Sally Kohn is an activist, columnist and television commentator, and a supporter of Bernie Sanders. Follow her on Twitter: @sallykohn . The opinions expressed in this commentary are hers.

(CNN) If Hillary Clinton is the Democratic nominee for president, she should find a substantive and valuable role for Bernie Sanders. It should not be as her vice president.

Before Sanders supporters take my head off, let me explain:

I'm a Bernie Sanders supporter and believe wholeheartedly in his economic populist vision for the Democratic Party and for America, but he doesn't do what a VP candidate needs to do: help the presidential ticket win votes.

Sanders has given small signals that he'd be willing to step into that job (on "Meet the Press" last weekend, he wouldn't rule it out) , and there's little doubt that such a move would help Clinton secure Sanders' voters. Beyond that, at least one recent poll (from not-always-reliable Rasmussen) shows Democrats preferring Sanders as a VP pick.

But this should not be the main factor influencing Clinton's VP decision. Bear in mind: Back in 2008, when Hillary Clinton was in a primary race with Barack Obama, mid-May exit polls showed fully half of her supporters in Indiana saying they would not vote for Barack Obama if he was the Democratic nominee. And one-third of Clinton supporters said they would actually back John McCain over Obama. North Carolina polling numbers back then told the same story.

Guess what happened? In the general election, Obama beat McCain in both states. Those voters came around.

And that was only in a match-up against relative Republican moderate John McCain. It's hard to imagine that Democrats, however bitter they may feel about the primary battle, won't unite against Donald Trump.

What's more, Sanders' potential benefits as a VP pick may be offset by his downside -- namely that he makes the Clinton ticket look even more elderly.

So if not Bernie, who?

JUST WATCHED Clinton adds CA campaigning to fend off Sanders Replay More Videos ... MUST WATCH Clinton adds CA campaigning to fend off Sanders 03:40

If Clinton is genuinely worried about exciting Sanders' base and looking to address this with her VP pick, Elizabeth Warren is a much stronger choice. Warren would bring the same base-rallying energy as Sanders, plus younger energy and something else very useful in a running mate: a demonstrated ability to effectively attack Trump -- particularly on his Twitter turf -- something Clinton has been either unwilling or unable to do so far.

Plus, Warren would directly tap into the overall anti-establishment moment coursing through the electorate.

There are some other baldly pragmatic reasons that Clinton must look beyond Sanders for her running mate -- whether to Warren or someone else -- in what could be a painfully close general election.

All signs point to a tough battle in Rust Belt states such as Pennsylvania, Ohio, Wisconsin and Michigan, crucial to this election's outcome. A Democratic nominee needs a VP candidate who, yes, aligns with her (or his) values and vision for the nation -- but also helps pick up these key states and the demographics within them.

JUST WATCHED Sanders campaign: Wait for super delegates to vote Replay More Videos ... MUST WATCH Sanders campaign: Wait for super delegates to vote 06:51

Consider Ohio and Pennsylvania -- the first- and second-most competitive battleground states in 2008, respectively. The conventional media narrative is that these states reflect the concerns of their white voting populations, that winning over this demographic is key to winning the general election. And for Donald Trump that would surely be true, as his path to victory runs straight through white male voters, as a group.

But in 2012, Barack Obama won Ohio because African-American voters turned out at an even greater percentage than in 2008, giving him a sizable margin. There's a reason the limited demographic appeal of Sanders stings his supporters: The reality is that Democrats depend not on white swing voters but on mobilizing and activating voters of color (who have been slow to warm to Sanders) to win a national election.

In this sense, HUD Secretary Julian Castro, a Latino, or former Massachusetts Gov. Deval Patrick, an African-American, would be a good choice for Clinton's VP. Sen. Cory Booker of New Jersey certainly would add youthful energy to the ticket, though his record of accomplishment is sparser. Tom Perez, secretary of labor under Obama, is also a strong choice because he would help motivate Latino voter turnout and appeal to the Sanders wing of the party.

It is possible, of course, that the centrism Clinton is known for would rear its head in her vice presidential choice.

Consider this: Faced with a general election against Donald Trump, an opponent that many decent Republicans find repugnant, Clinton and her team may use the VP pick to appeal to moderate swing voters and even some Republicans -- thereby expanding her base beyond those left-of-center white, Latino and black voters she can already count on.

In this scenario, might she try to woo Tom Ridge, the former governor of Pennsylvania and the director of Homeland Security under George W. Bush? He's a pro-choice Republican and -- by his party's standards today -- a moderate.

Or Sen. Sherrod Brown of Ohio, who could help with the Rust Belt while helping with the Sanders vote, since Brown is considered more progressive than Clinton.

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

Or -- a very safe choice -- Tim Kaine, a Democratic U.S. senator and former governor of Virginia with impeccable centrist Democratic establishment credentials, well-tested and well-liked, and a solid pick for a heartbeat-away job.

Ultimately, if (and remember, it's still if) Hillary Clinton is the Democratic Party nominee, Sanders and Warren have already demonstrated a clear willingness to support her nomination and help defeat Trump. As the stakes are raised, one can expect their commitment to be raised as well. And certainly Clinton could look to Obama's model and promise a senior cabinet post to Sanders as a significant olive branch.

No question, it's going to be tricky picking a person capable of threading all these needles -- a person who engages and even excites the Sanders base and reflects the anti-establishment populist zeitgeist of the moment on left and right, draws in swing voters and resentful Republican votes and has the personality to challenge Trump head-on while also clearly possessing the temperament and qualifications of the presidency.

Easy, right?