(CNN) Hillary Clinton said Tuesday she opposes the controversial Keystone XL pipeline, taking sides with progressives who are fighting the 1,179-mile project over environmental concerns.

The announcement, which comes after months of Clinton remaining mum over the hot-button 2016 issue, immediately drew praise from liberals and environmental groups but was criticized by Republican presidential candidates.

"I think it is imperative that we look at the Keystone pipeline as what I believe it is -- a distraction from important work we have to do on climate change," Clinton told a community forum in Des Moines, Iowa.

"And unfortunately from my perspective, one that interferes with our ability to move forward with all the other issues," she said. "Therefore I oppose it."

The Democratic 2016 front-runner announced her opposition to the project -- which is still the subject of a years-long State Department review -- as Pope Francis landed in the United States, dominating national media attention.

Clinton had not previously disclosed her position on the campaign trail despite consistent questions about her position on the project, which is widely favored by conservatives but opposed by liberals who believe it will contribute to climate change. In explaining her answer Tuesday, Clinton said she didn't want to interfere with a review process that started under her watch.

"I was in a unique position as secretary of state at the start of this process, and not wanting to interfere with ongoing decision-making that the President and Secretary (of State John) Kerry have to do in order to make whatever final decisions they need," Clinton said. "So I thought this would be decided by now, and therefore I could tell you whether I agree or disagree, but it hasn't been decided, and I feel now I've got a responsibility to you and voters who ask me about this."

Speaking to the Des Moines Register's editorial board after the event, Clinton said she had "no idea" she would be asked about the pipeline Tuesday.

But, she said, "I think I owed it to people to say where I stood," adding, "clearly, the time had come for me to answer the question."

Jennifer Palmieri, Clinton's communications director, said in a statement to CNN that Clinton's role as a former secretary of state put her "in a different situation than other candidates."

Photos: Hillary Clinton's life in the spotlight Photos: Hillary Clinton's life in the spotlight 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. Hide Caption 1 of 43 Photos: Hillary Clinton's life in the spotlight 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. Hide Caption 2 of 43 Photos: Hillary Clinton's life in the spotlight Rodham was a lawyer on the House Judiciary Committee, whose work led to impeachment charges against President Richard Nixon in 1974. Hide Caption 3 of 43 Photos: Hillary Clinton's life in the spotlight 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. Hide Caption 4 of 43 Photos: Hillary Clinton's life in the spotlight Arkansas' first lady, now using the name Hillary Rodham Clinton, wears her inaugural ball gown in 1985. Hide Caption 5 of 43 Photos: Hillary Clinton's life in the spotlight The Clintons celebrate Bill's inauguration in Little Rock, Arkansas, in 1991. He was governor from 1983 to 1992, when he was elected President. Hide Caption 6 of 43 Photos: Hillary Clinton's life in the spotlight 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. Hide Caption 7 of 43 Photos: Hillary Clinton's life in the spotlight 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. Hide Caption 8 of 43 Photos: Hillary Clinton's life in the spotlight During the 1992 presidential campaign, Clinton jokes with her husband's running mate, Al Gore, and Gore's wife, Tipper, aboard a campaign bus. Hide Caption 9 of 43 Photos: Hillary Clinton's life in the spotlight Clinton accompanies her husband as he takes the oath of office in January 1993. Hide Caption 10 of 43 Photos: Hillary Clinton's life in the spotlight The Clintons share a laugh on Capitol Hill in 1993. Hide Caption 11 of 43 Photos: Hillary Clinton's life in the spotlight Clinton unveils the renovated Blue Room of the White House in 1995. Hide Caption 12 of 43 Photos: Hillary Clinton's life in the spotlight 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. Hide Caption 13 of 43 Photos: Hillary Clinton's life in the spotlight The Clintons hug as Bill is sworn in for a second term as President. Hide Caption 14 of 43 Photos: Hillary Clinton's life in the spotlight The first lady holds up a Grammy Award, which she won for her audiobook "It Takes a Village" in 1997. Hide Caption 15 of 43 Photos: Hillary Clinton's life in the spotlight 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. Hide Caption 16 of 43 Photos: Hillary Clinton's life in the spotlight 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. Hide Caption 17 of 43 Photos: Hillary Clinton's life in the spotlight The first family walks with their dog, Buddy, as they leave the White House for a vacation in August 1998. Hide Caption 18 of 43 Photos: Hillary Clinton's life in the spotlight 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. Hide Caption 19 of 43 Photos: Hillary Clinton's life in the spotlight Clinton announces in February 2000 that she will seek the U.S. Senate seat in New York. She was elected later that year. Hide Caption 20 of 43 Photos: Hillary Clinton's life in the spotlight Clinton makes her first appearance on the Senate Environment and Natural Resources Committee. Hide Caption 21 of 43 Photos: Hillary Clinton's life in the spotlight 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. Hide Caption 22 of 43 Photos: Hillary Clinton's life in the spotlight Clinton holds up her book "Living History" before a signing in Auburn Hills, Michigan, in 2003. Hide Caption 23 of 43 Photos: Hillary Clinton's life in the spotlight Clinton and another presidential hopeful, U.S. Sen. Barack Obama, applaud at the start of a Democratic debate in 2007. Hide Caption 24 of 43 Photos: Hillary Clinton's life in the spotlight 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. Hide Caption 25 of 43 Photos: Hillary Clinton's life in the spotlight 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. Hide Caption 26 of 43 Photos: Hillary Clinton's life in the spotlight Clinton, as secretary of state, greets Russian Prime Minister Vladimir Putin during a meeting just outside Moscow in March 2010. Hide Caption 27 of 43 Photos: Hillary Clinton's life in the spotlight The Clintons pose on the day of Chelsea's wedding to Marc Mezvinsky in July 2010. Hide Caption 28 of 43 Photos: Hillary Clinton's life in the spotlight 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. Hide Caption 29 of 43 Photos: Hillary Clinton's life in the spotlight 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. Hide Caption 30 of 43 Photos: Hillary Clinton's life in the spotlight 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. Hide Caption 31 of 43 Photos: Hillary Clinton's life in the spotlight 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. Hide Caption 32 of 43 Photos: Hillary Clinton's life in the spotlight 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. Hide Caption 33 of 43 Photos: Hillary Clinton's life in the spotlight Clinton, now running for President again, performs with Jimmy Fallon during a "Tonight Show" skit in September 2015. Hide Caption 34 of 43 Photos: Hillary Clinton's life in the spotlight 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. Hide Caption 35 of 43 Photos: Hillary Clinton's life in the spotlight 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." Hide Caption 36 of 43 Photos: Hillary Clinton's life in the spotlight Clinton is reflected in a teleprompter during a campaign rally in Alexandria, Virginia, in October 2015. Hide Caption 37 of 43 Photos: Hillary Clinton's life in the spotlight Clinton walks on her stage with her family after winning the New York primary in April. Hide Caption 38 of 43 Photos: Hillary Clinton's life in the spotlight 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." Hide Caption 39 of 43 Photos: Hillary Clinton's life in the spotlight 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. Hide Caption 40 of 43 Photos: Hillary Clinton's life in the spotlight 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. Hide Caption 41 of 43 Photos: Hillary Clinton's life in the spotlight 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. Hide Caption 42 of 43 Photos: Hillary Clinton's life in the spotlight 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. Hide Caption 43 of 43

"Having the experience of being a former secretary of state distinguishes her and her candidacy, but it comes with responsibilities that at times can limit her," Palmieri said. "But we know that the experience is well worth whatever price she may pay politically."

A Clinton campaign aide told CNN that the former secretary of state couldn't wait any longer to explain her position.

"She's been taking on water for (not taking a position) ... She didn't want to jam Secretary Kerry or jam the President but it was just time. It's September," the aide said.

The aide said as pressure had mounted for Clinton to take a position, she wanted to give the administration space but doing so became untenable. The aide noted Clinton's meeting with the Des Moines Register, and the campaign was expecting the question to come up. She wanted to be able to answer, the aide said.

The White House was briefed on Clinton's position prior to her comments Tuesday, another Clinton aide said.

"Also, in the course of discussing her plans for increasing investment in energy infrastructure with labor officials in recent weeks, she privately made her opposition to the pipeline known to them as well," the aide added.

Clio Cullison, a student at Drake University who came to the event after a friend of hers at 350.org, an active climate change advocacy group that has regularly followed Clinton on the campaign trail, asked her to attend and ask Clinton about the pipeline.

"I was really nervous to ask," Cullison told CNN. "I haven't asked any political candidates a question ever, so that was really exciting."

The student added that she "was afraid of her answer, to be honest. I didn't know where she was going to stand. I didn't know if she was going to answer at all. I am really glad she did answer, one, and two, did oppose the Keystone pipeline."

A frequent question on the trail

Clinton has repeatedly been asked about Keystone on the campaign trail but has never answered directly.

"I am not going to second guess (President Barack Obama) because I was in a position to set this in motion," Clinton said at a July event in New Hampshire. "I want to wait and see what he and Secretary Kerry decide."

At the same event, she later added, "If it is undecided when I become president, I will answer your question."

And throughout much of 2013 and 2014, Clinton criss-crossed the country on the paid speaking circuit and later on her book tour. She was asked about Keystone a number of times, particularly in Canada, where the pipeline would originate. At no point did she take a position, however.

JUST WATCHED Hillary Clinton won't answer Keystone XL pipeline question Replay More Videos ... MUST WATCH Hillary Clinton won't answer Keystone XL pipeline question 00:30

Clinton's announcement on Tuesday was met with praise from environmental groups.

Jane Kleeb, director of the anti-pipeline group Bold Nebraska, said the decision "was a long time coming," and demonstrates that Democratic candidates need to pay closer attention to the progressive base.

"Political insiders continue to not give credit to the climate movement and not give credit to farmers and ranchers who are opposed to these risky fossil fuel projects," Kleeb told CNN. "This is a big part of her progressive base -- people who are not just against Keystone but want to see action on climate change."

And Bill McKibben, co-founder of 350.org, said Clinton has slowly been moving in this direction since 2010, when she said she was "inclined" to approve the project. "It's been a good evolution, always in the right direction," he said.

"Over time, she has come to understand that a defining issue of the next election is climate change and there's no way to address it seriously without this being answered," McKibben said, calling it a "boondoggle" that he expects Obama to reject as well.

2016ers weigh in

Clinton's Democratic presidential opponents have opposed the deal. On Tuesday, former Maryland Gov. Martin O'Malley, lambasted her for the delay in taking a position.

"On issue after issue -- marriage equality, drivers licenses for undocumented immigrants, children fleeing violence in Central America, the Syrian refugee crisis, and now the Keystone Pipeline, Secretary Clinton has followed -- not forged -- public opinion," O'Malley said in a statement.

Vermont Sen. Bernie Sanders said he was "glad" Clinton came out against the pipeline.

"As a senator who has vigorously opposed the Keystone pipeline from the beginning, I am glad that Secretary Clinton finally has made a decision and I welcome her opposition to the pipeline," Sanders said. "Clearly it would be absurd to encourage the extraction and transportation of some of the dirtiest fossil fuel on the planet."

But Republican presidential hopefuls quickly bashed Clinton over the announcement. Jeb Bush slammed Clinton for favoring "environmental extremists" in making her decision.

".@HillaryClinton finally says what we already knew. She favors environmental extremists over U.S. jobs. #KeystoneXL," he tweeted.

.@HillaryClinton finally says what we already knew. She favors environmental extremists over U.S. jobs. #KeystoneXL https://t.co/KOhVPPj8xm — Jeb Bush (@JebBush) September 22, 2015

Bobby Jindal noted that Clinton's announcement came at the same time Pope Francis arrived in the U.S.

"Hoping that Americans would be distracted by the Pope's visit, Hillary finally admitted she opposes #KeystoneXL," Jindal tweeted, linking to a petition on his campaign website to urge construction of the pipeline.

Hoping that Americans would be distracted by the Pope's visit, Hillary finally admitted she opposes #KeystoneXL. https://t.co/JgUN43puWU — Gov. Bobby Jindal (@BobbyJindal) September 22, 2015

South Carolina Sen. Lindsey Graham fired off a series of tweets, saying the pipeline would help the economy and boost national security by reducing dependence on foreign oli.

"In opposing Keystone pipeline, @HillaryClinton once again shows that she intends to continue the failed polices of the Obama Administration," he said.