Washington (CNN) As Hillary Clinton returns to Iowa on Monday, a sense of anxiety is cascading through her campaign, with an increasing sense of urgency the primary fight with Bernie Sanders is far more of a threat than once imagined, unlikely to be extinguished after the first contests in Iowa and New Hampshire.

Clinton has gone from all but ignoring Sanders to fiercely engaging him in recent days, a reflection of public and private polling that points to a race that is uncomfortably competitive for the Democratic front-runner. She openly questions his electability and argues that he is out of step with the party on guns and other issues.

After conceding the presidency to Trump in a phone call earlier, Clinton addresses supporters and campaign workers in New York on Wednesday, November 9. Her defeat marked a stunning end to a campaign that appeared poised to make her the first woman elected US president.

Clinton addresses a campaign rally in Cleveland on November 6, two days before Election Day. She went on to lose Ohio -- and the election -- to her Republican opponent, Donald Trump.

Clinton addresses a campaign rally in Cleveland on November 6, two days before Election Day. She went on to lose Ohio -- and the election -- to her Republican opponent, Donald Trump.

Clinton arrives at a 9/11 commemoration ceremony in New York on September 11. Clinton, who was diagnosed with pneumonia two days before, left early after feeling ill. A video appeared to show her stumble as Secret Service agents helped her into a van.

Obama hugs Clinton after he gave a speech at the Democratic National Convention in Philadelphia. The president said Clinton was ready to be commander in chief. "For four years, I had a front-row seat to her intelligence, her judgment and her discipline," he said, referring to her stint as his secretary of state.

Obama hugs Clinton after he gave a speech at the Democratic National Convention in Philadelphia. The president said Clinton was ready to be commander in chief. "For four years, I had a front-row seat to her intelligence, her judgment and her discipline," he said, referring to her stint as his secretary of state.

After Clinton became the Democratic Party's presumptive nominee, this photo was posted to her official Twitter account. "To every little girl who dreams big: Yes, you can be anything you want -- even president," Clinton said. "Tonight is for you."

After Clinton became the Democratic Party's presumptive nominee, this photo was posted to her official Twitter account. "To every little girl who dreams big: Yes, you can be anything you want -- even president," Clinton said. "Tonight is for you."

Clinton walks on her stage with her family after winning the New York primary in April.

Clinton walks on her stage with her family after winning the New York primary in April.

Clinton is reflected in a teleprompter during a campaign rally in Alexandria, Virginia, in October 2015.

Clinton is reflected in a teleprompter during a campaign rally in Alexandria, Virginia, in October 2015.

U.S. Sen. Bernie Sanders shares a lighthearted moment with Clinton during a Democratic presidential debate in October 2015. It came after Sanders gave his take on the Clinton email scandal. "The American people are sick and tired of hearing about the damn emails," Sanders said. "Enough of the emails. Let's talk about the real issues facing the United States of America."

U.S. Sen. Bernie Sanders shares a lighthearted moment with Clinton during a Democratic presidential debate in October 2015. It came after Sanders gave his take on the Clinton email scandal. "The American people are sick and tired of hearing about the damn emails," Sanders said. "Enough of the emails. Let's talk about the real issues facing the United States of America."

Clinton testifies about the Benghazi attack during a House committee meeting in October 2015. "I would imagine I have thought more about what happened than all of you put together," she said during the 11-hour hearing. "I have lost more sleep than all of you put together. I have been wracking my brain about what more could have been done or should have been done." Months earlier, Clinton had acknowledged a "systemic breakdown" as cited by an Accountability Review Board, and she said that her department was taking additional steps to increase security at U.S. diplomatic facilities.

Clinton testifies about the Benghazi attack during a House committee meeting in October 2015. "I would imagine I have thought more about what happened than all of you put together," she said during the 11-hour hearing. "I have lost more sleep than all of you put together. I have been wracking my brain about what more could have been done or should have been done." Months earlier, Clinton had acknowledged a "systemic breakdown" as cited by an Accountability Review Board, and she said that her department was taking additional steps to increase security at U.S. diplomatic facilities.

Clinton, now running for President again, performs with Jimmy Fallon during a "Tonight Show" skit in September 2015.

Clinton, now running for President again, performs with Jimmy Fallon during a "Tonight Show" skit in September 2015.

Clinton ducks after a woman threw a shoe at her while she was delivering remarks at a recycling trade conference in Las Vegas in 2014.

Clinton ducks after a woman threw a shoe at her while she was delivering remarks at a recycling trade conference in Las Vegas in 2014.

Obama and Clinton bow during the transfer-of-remains ceremony marking the return of four Americans, including U.S. Ambassador Christopher Stevens, who were killed in Benghazi, Libya, in September 2012.

Obama and Clinton bow during the transfer-of-remains ceremony marking the return of four Americans, including U.S. Ambassador Christopher Stevens, who were killed in Benghazi, Libya, in September 2012.

Clinton arrives for a group photo before a forum with the Gulf Cooperation Council in March 2012. The forum was held in Riyadh, Saudi Arabia.

Clinton arrives for a group photo before a forum with the Gulf Cooperation Council in March 2012. The forum was held in Riyadh, Saudi Arabia.

Clinton checks her Blackberry inside a military plane after leaving Malta in October 2011. In 2015, The New York Times reported that Clinton exclusively used a personal email account during her time as secretary of state. The account, fed through its own server, raises security and preservation concerns. Clinton later said she used a private domain out of "convenience," but admits in retrospect "it would have been better" to use multiple emails.

Clinton checks her Blackberry inside a military plane after leaving Malta in October 2011. In 2015, The New York Times reported that Clinton exclusively used a personal email account during her time as secretary of state. The account, fed through its own server, raises security and preservation concerns. Clinton later said she used a private domain out of "convenience," but admits in retrospect "it would have been better" to use multiple emails.

In this photo provided by the White House, Obama, Clinton, Biden and other members of the national security team receive an update on the mission against Osama bin Laden in May 2011.

In this photo provided by the White House, Obama, Clinton, Biden and other members of the national security team receive an update on the mission against Osama bin Laden in May 2011.

The Clintons pose on the day of Chelsea's wedding to Marc Mezvinsky in July 2010.

The Clintons pose on the day of Chelsea's wedding to Marc Mezvinsky in July 2010.

Clinton, as secretary of state, greets Russian Prime Minister Vladimir Putin during a meeting just outside Moscow in March 2010.

Clinton, as secretary of state, greets Russian Prime Minister Vladimir Putin during a meeting just outside Moscow in March 2010.

Obama is flanked by Clinton and Vice President-elect Joe Biden at a news conference in Chicago in December 2008. He had designated Clinton to be his secretary of state.

Obama is flanked by Clinton and Vice President-elect Joe Biden at a news conference in Chicago in December 2008. He had designated Clinton to be his secretary of state.

Obama and Clinton talk on the plane on their way to a rally in Unity, New Hampshire, in June 2008. She had recently ended her presidential campaign and endorsed Obama.

Obama and Clinton talk on the plane on their way to a rally in Unity, New Hampshire, in June 2008. She had recently ended her presidential campaign and endorsed Obama.

Clinton and another presidential hopeful, U.S. Sen. Barack Obama, applaud at the start of a Democratic debate in 2007.

Clinton and another presidential hopeful, U.S. Sen. Barack Obama, applaud at the start of a Democratic debate in 2007.

Clinton holds up her book "Living History" before a signing in Auburn Hills, Michigan, in 2003.

Clinton holds up her book "Living History" before a signing in Auburn Hills, Michigan, in 2003.

Sen. Clinton comforts Maren Sarkarat, a woman who lost her husband in the September 11 terrorist attacks, during a ground-zero memorial in October 2001.

Sen. Clinton comforts Maren Sarkarat, a woman who lost her husband in the September 11 terrorist attacks, during a ground-zero memorial in October 2001.

Clinton makes her first appearance on the Senate Environment and Natural Resources Committee.

Clinton makes her first appearance on the Senate Environment and Natural Resources Committee.

Clinton announces in February 2000 that she will seek the U.S. Senate seat in New York. She was elected later that year.

Clinton announces in February 2000 that she will seek the U.S. Senate seat in New York. She was elected later that year.

President Clinton makes a statement at the White House in December 1998, thanking members of Congress who voted against his impeachment. The Senate trial ended with an acquittal in February 1999.

President Clinton makes a statement at the White House in December 1998, thanking members of Congress who voted against his impeachment. The Senate trial ended with an acquittal in February 1999.

The first family walks with their dog, Buddy, as they leave the White House for a vacation in August 1998.

The first family walks with their dog, Buddy, as they leave the White House for a vacation in August 1998.

Clinton looks on as her husband discusses the Monica Lewinsky scandal in the Roosevelt Room of the White House on January 26, 1998. Clinton declared, "I did not have sexual relations with that woman." In August of that year, Clinton testified before a grand jury and admitted to having "inappropriate intimate contact" with Lewinsky, but he said it did not constitute sexual relations because they had not had intercourse. He was impeached in December on charges of perjury and obstruction of justice.

Clinton looks on as her husband discusses the Monica Lewinsky scandal in the Roosevelt Room of the White House on January 26, 1998. Clinton declared, "I did not have sexual relations with that woman." In August of that year, Clinton testified before a grand jury and admitted to having "inappropriate intimate contact" with Lewinsky, but he said it did not constitute sexual relations because they had not had intercourse. He was impeached in December on charges of perjury and obstruction of justice.

The Clintons dance on a beach in the U.S. Virgin Islands in January 1998. Later that month, Bill Clinton was accused of having a sexual relationship with former White House intern Monica Lewinsky.

The Clintons dance on a beach in the U.S. Virgin Islands in January 1998. Later that month, Bill Clinton was accused of having a sexual relationship with former White House intern Monica Lewinsky.

The first lady holds up a Grammy Award, which she won for her audiobook "It Takes a Village" in 1997.

The first lady holds up a Grammy Award, which she won for her audiobook "It Takes a Village" in 1997.

The Clintons hug as Bill is sworn in for a second term as President.

The Clintons hug as Bill is sworn in for a second term as President.

Clinton waves to the media in January 1996 as she arrives for an appearance before a grand jury in Washington. The first lady was subpoenaed to testify as a witness in the investigation of the Whitewater land deal in Arkansas. The Clintons' business investment was investigated, but ultimately they were cleared of any wrongdoing.

Clinton waves to the media in January 1996 as she arrives for an appearance before a grand jury in Washington. The first lady was subpoenaed to testify as a witness in the investigation of the Whitewater land deal in Arkansas. The Clintons' business investment was investigated, but ultimately they were cleared of any wrongdoing.

Clinton unveils the renovated Blue Room of the White House in 1995.

Clinton unveils the renovated Blue Room of the White House in 1995.

Clinton accompanies her husband as he takes the oath of office in January 1993.

Clinton accompanies her husband as he takes the oath of office in January 1993.

During the 1992 presidential campaign, Clinton jokes with her husband's running mate, Al Gore, and Gore's wife, Tipper, aboard a campaign bus.

During the 1992 presidential campaign, Clinton jokes with her husband's running mate, Al Gore, and Gore's wife, Tipper, aboard a campaign bus.

In June 1992, Clinton uses a sewing machine designed to eliminate back and wrist strain. She had just given a speech at a convention of the International Ladies' Garment Workers Union.

In June 1992, Clinton uses a sewing machine designed to eliminate back and wrist strain. She had just given a speech at a convention of the International Ladies' Garment Workers Union.

Bill Clinton comforts his wife on the set of "60 Minutes" after a stage light broke loose from the ceiling and knocked her down in January 1992.

Bill Clinton comforts his wife on the set of "60 Minutes" after a stage light broke loose from the ceiling and knocked her down in January 1992.

The Clintons celebrate Bill's inauguration in Little Rock, Arkansas, in 1991. He was governor from 1983 to 1992, when he was elected President.

The Clintons celebrate Bill's inauguration in Little Rock, Arkansas, in 1991. He was governor from 1983 to 1992, when he was elected President.

Arkansas' first lady, now using the name Hillary Rodham Clinton, wears her inaugural ball gown in 1985.

Arkansas' first lady, now using the name Hillary Rodham Clinton, wears her inaugural ball gown in 1985.

In 1975, Rodham married Bill Clinton, whom she met at Yale Law School. He became the governor of Arkansas in 1978. In 1980, the couple had a daughter, Chelsea.

In 1975, Rodham married Bill Clinton, whom she met at Yale Law School. He became the governor of Arkansas in 1978. In 1980, the couple had a daughter, Chelsea.

Rodham was a lawyer on the House Judiciary Committee, whose work led to impeachment charges against President Richard Nixon in 1974.

Rodham was a lawyer on the House Judiciary Committee, whose work led to impeachment charges against President Richard Nixon in 1974.

Before marrying Bill Clinton, she was Hillary Rodham. Here she attends Wellesley College in Massachusetts. Her commencement speech at Wellesley's graduation ceremony in 1969 attracted national attention. After graduating, she attended Yale Law School.

Before marrying Bill Clinton, she was Hillary Rodham. Here she attends Wellesley College in Massachusetts. Her commencement speech at Wellesley's graduation ceremony in 1969 attracted national attention. After graduating, she attended Yale Law School.

Hillary Clinton accepts the Democratic Party's nomination for president at the Democratic National Convention in Philadelphia on July 28, 2016. The former first lady, U.S. senator and secretary of state was the first woman to lead the presidential ticket of a major political party.

Hillary Clinton accepts the Democratic Party's nomination for president at the Democratic National Convention in Philadelphia on July 28, 2016. The former first lady, U.S. senator and secretary of state was the first woman to lead the presidential ticket of a major political party.

But there are fresh questions about her electability argument, with Sanders faring better than Clinton in a series of hypothetical head-to-head matchups with top Republicans in Iowa and New Hampshire, according to the latest NBC/Wall Street Journal/Marist poll. Sanders holds a double-digit edge over Trump in both states, for example, while Clinton has a far narrower lead.

Still, Clinton is hoping to persuade voters who may be intrigued by Sanders' populist rage to take a longer view and embrace her candidacy, whether or not they were initially enthused.

Kathleen Jurgens, an Iowa Democrat, walked into a Clinton rally this week in Council Bluffs torn between the two. But an hour later, she said she was sold on Clinton's pitch that she was the only candidate who could stop a Republican from winning the White House.

"I really like Bernie. He's outspoken and he doesn't seem as political," Jurgens told CNN. "But at this point, you really have to look at electability and Hillary really impressed me."

The unknown question causing heartburn for Team Clinton, from its campaign headquarters in Brooklyn to offices across Iowa and New Hampshire, is how many Democrats will join Jurgens and how many will stay loyal to Sanders in a year when an anti-establishment sentiment is coursing through the electorate.

The persistent challenge from Sanders presents a critical challenge for Clinton. She's in a far stronger position in many states that hold primaries later in the nominating contest. But a loss in the Iowa caucuses on February 1 will will revive memories of Clinton's third-place finish there in 2008, which ultimately doomed her first presidential bid. She's facing an even tougher race in New Hampshire, where she won the crucial primary eight years ago.

Clinton herself penned a fundraising email on Saturday that indicated the "primary race in New Hampshire is neck and neck."

While the message was designed to lower the campaign's expectations, it also underscored the sense of worry as the voting in the opening two contests are less than a month away.

"A loss there could be a sharp blow to all the work we've done. We've got the momentum to carry this win, but our folks on the ground still need the resources to put us over the finish line," Clinton wrote. "There's a lot riding on the New Hampshire primary in just one month. It's going to be close, but I'm ready to do this."

Clinton has struggled to match the enthusiasm generated by Sanders, from building a grassroots fundraising network to drawing large crowds. She has also had trouble creating a sense of urgency among her committed supporters, which is one reason she is escalating her exchanges with Sanders.

This week, Clinton and her aides suggested Sanders was out-of-step with the Democratic Party on a host of issues. The Clinton campaign has either responded -- or attacked Sanders first -- on Wall Street reform, his ability to win in November, his support for paid family leave and gun control measures.

For months, Clinton's campaign declined to get into regular exchanges with Sanders and his aides. Clinton herself waited months to even say his name on the campaign trail. The offensive stance was on full display on Friday night, when Clinton and her aides let loose on Sanders on guns.

"When it really mattered, Senator Sanders voted with the gun lobby and I voted against the gun lobby," Clinton said, taking the unusual step of calling into MSNBC's "Hardball."

She accused Sanders of saying one thing and doing another and called on him to "introduce legislation to repeal the immunity that was given to gun makers and sellers."

Aides acknowledge Clinton has not closed the sale in Iowa or New Hampshire. The energy that propelled Sanders through the summer has not diminished, which is a potential sign of worry for Clinton. Bill Clinton is returning to New Hampshire and Chelsea Clinton is making her campaign debut there next week.

A new poll released Sunday showed a tight race in Iowa, with Clinton holding a three-point lead among likely voters over Sanders, 48-45, according to the NBC/WSJ/Marist survey . In New Hampshire, the poll found Sanders ahead of Clinton by four points, 50-46, among likely primary voters.

"No wonder they're in attack mode," Michael Briggs, Sanders' spokesman, said after the heated week. "Secretary Clinton and her team are getting nervous and nasty because the so-called inevitable nominee anointed by the establishment eight months ago doesn't look so inevitable anymore."

While Sanders aides expected the race to intensify, the sudden escalation surprised some advisers after months of a largely timid back-and-forth, particularly compared to the raucous Republican contest.

In Iowa and New Hampshire this week, she subtly slammed Sanders on his ability to keep the White House out of Republican hands.

"Think hard about the people who are presenting themselves to you, their experience, their qualifications, their positions. And particularly for those of us who are Democrats, their electability," Clinton told Iowa voters. "And how we make sure we have a Democrat going back into that White House on January 20, 2017."

It's an open question whether Clinton's promise of electability -- reminiscent of the argument she made eight years ago against Barack Obama -- will be embraced or backfire.

The first test is February 1 at the Iowa caucuses, followed a week later in the New Hampshire primary. The length of the Democratic primary fight from there depends on the opening two acts.

CORRECTION: An earlier version of this story misstated the results of the December Des Moines Register poll.