NOEL KING, HOST:
Twelve Democratic presidential candidates will debate tonight in Westerville, Ohio. For a race that seemed static for a long time, suddenly there is a lot going on. Here's NPR national political correspondent Mara Liasson.
MARA LIASSON, BYLINE: For the first time since he entered the race, Joe Biden has lost his clear front-runner position. Elizabeth Warren is on the rise, and Bernie Sanders just had a heart attack, so you wouldn't blame Democrats if their heads are spinning.
DAVID AXELROD: If you look at the Democratic race now, you'd have to say that Elizabeth Warren is the stock that you would buy. Biden may be stock that you hold on to for a while longer, but you're getting nervous.
LIASSON: That's David Axelrod, who was Barack Obama's top strategist. Democrats are nervous because Biden is being pummeled by millions of dollars of Trump attack ads pushing a false narrative that Biden and his son did something illegal in Ukraine when Biden was the vice president. That, says Axelrod, is the 800-pound gorilla that will certainly come up in Ohio tonight.
AXELROD: Most of what the president has said has been widely debunked. But there is this question of, you know, why his son was on the payroll of this Ukrainian company while he was vice president. I don't know that he can get away with swatting that question aside as he has.
LIASSON: Democrats are worried that even though the Trump charges are false, the president will get a kind of liars dividend. His multimillion dollars' worth of attack ads on TV and social media amplify the charges and sow doubt among Democrats, not that Biden is guilty but that he'll be weakened as a candidate. Still, the attacks are an opportunity for Biden, says Tara McGowan the, founder of Acronym, a Democratic digital advocacy organization.
TARA MCGOWAN: I think that this provides Biden potentially the best opportunity they're ever going to get to set up this real fight between him and Trump and demonstrate his strength and electability against Trump in the general election.
LIASSON: After wrestling with how best to handle Trump's attacks, last week Biden decided to get more aggressive.
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JOE BIDEN: We all laughed when he said he could stand in the middle of Fifth Avenue and shoot someone and get away with it. It's no joke. He's shooting holes in the Constitution. And we cannot let him get away with it.
LIASSON: While Biden has been figuring out how to handle Trump, Elizabeth Warren has continued her steady rise - and not just because she's run a really good primary campaign with money, organization and grassroots enthusiasm but also, says Democratic pollster Stan Greenberg, because she tapped into a pent-up desire among Democrats for government action to solve big problems like corporate corruption, income inequality and climate change.
STAN GREENBERG: She is constantly talking about things government can do. Democrats want an activist government. And they're looking for it after being suppressed by a decade of Tea Party dominance and now Trump dominance.
LIASSON: Warren stands to benefit if Biden can't jujitsu Trump's attacks to strengthen his own position. She also stands to benefit if Sanders' heart condition keeps him on the sidelines. But with Warren's rise comes scrutiny, which she has not had to face so far. As one centrist Democrat quipped, Warren has a plan for everything except how she'll beat Donald Trump. And that's another thing Democrats are worried about, says Tara McGowan.
MCGOWAN: How she is going to get the most voters, including independents and swing voters who don't want to see another four years of Trump but maybe aren't as excited about the more progressive policies that she champions. That really is the challenge.
LIASSON: Worries among Democrats that Warren is too far left to win a general election spiked last week when Warren gave this flip response on CNN when asked by an LGBTQ activist what she would say to someone who believes marriage is between one man and one woman.
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ELIZABETH WARREN: And I'm going to say, then just marry one woman.
LIASSON: She waited a bit and then added this snarky comment.
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WARREN: Assuming you can find one.
LIASSON: Sounds like she was saying anyone with those beliefs is a loser - or maybe just deplorable. In addition to worries about Warren's electability and Biden's ability to perform, Democrats are also watching an intramural primary going on. You could call it the Biden understudy battle. That's where Pete Buttigieg is the candidate to watch, says David Axelrod. Buttigieg is still polling in the single digits, but he came in third in last quarter's fundraising, behind Warren and Sanders but ahead of Biden.
AXELROD: If Biden were to stumble, who could take up that lane? Pete Buttigieg is clearly now angling for that. He is more moderate in temperament and in policy certainly than Warren or Sanders. And he's been more pronounced about his differences with them lately.
LIASSON: So tonight's debate takes place against a backdrop of a lot of churn in the Democratic race. With 3 1/2 months before any Democrat votes or caucuses, a lot can still change.
Mara Liasson, NPR News, Washington. Transcript provided by NPR, Copyright NPR.