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At tonight's Democratic presidential debate in Houston, all the top contenders will be on the same stage - all 10. Of the more than 20 who declared, some have withdrawn, and others failed to qualify for this round. The candidates not onstage are devastated. The candidates who are onstage face opportunities - and also risks. Here's NPR national political correspondent Mara Liasson.
MARA LIASSON, BYLINE: There's been a big clash in every democratic debate so far. And not surprisingly, Joe Biden, the front-runner, has always been on the receiving end. The big question for tonight is whether Biden will be attacked by the candidate who is quickly emerging as his top challenger, Elizabeth Warren. Adam Green is the co-founder of the Progressive Change Campaign Committee, which supports Warren.
ADAM GREEN: The big pools of support for anyone to pick up votes are basically undecided voters and Joe Biden voters, all of whom are fundamentally electability voters who want to win.
LIASSON: So tonight, expect to hear Warren address herself to those voters who just want to win. On the campaign trail, she's been telling Democrats they don't have to choose a candidate they don't believe in just because they're scared of Donald Trump. Translation: Democrats, you really aren't that enthusiastic about Joe Biden, are you?
Tonight will also be the first chance for Warren and Biden to make a face-to-face ideological contrast. And it might come up around health care.
(SOUNDBITE OF ARCHIVED RECORDING)
JOE BIDEN: ...Is build on Obamacare, to build on what we did...
BIDEN: ...And secondly - secondly, to make sure that everyone does have an option.
ELIZABETH WARREN: I'm with Bernie on "Medicare for All," and let me tell you why. I spent a big chunk of my life studying why families go broke. And one of the No. 1 reasons is the cost of health care - medical bills.
LIASSON: Biden and Warren are on opposite sides of the Democratic debate about how to achieve universal health insurance coverage - build on Obamacare with a voluntary Medicare buy-in, the so-called public option; or scrap Obamacare, do away with private health insurance and create mandatory Medicare for All. Neera Tanden of the Democratic think tank Center for American Progress thinks that position is risky.
NEERA TANDEN: So I think on the issue of health care, the majority of the Democratic electorate and the majority of candidates have health care plans that would ensure that people who have private insurance through their employer are able to keep that choice.
LIASSON: Progressives who back mandatory Medicare for All say if they get a chance to explain that in place of private insurance, people will have no co-pays, premiums or deductibles, they can win the argument. But in the latest NPR/PBS NewsHour/Marist Poll, mandatory Medicare for All is unpopular with general election voters by a wide margin - 54-to-41. Other positions taken by Democratic candidates, including decriminalizing illegal border crossings and providing taxpayer-funded health insurance to undocumented immigrants, are also unpopular by 2-1 margins.
Republican consultant Rob Stutzman says all these positions are hurting the image of the Democratic Party overall and potentially helping Donald Trump.
ROB STUTZMAN: Some of these positions fits into the Trump playbook of being able to weaponize issues that sound risky. It really gives Trump an opportunity to go on offense.
LIASSON: But that's a problem for the general election. Right now, Democrats like Elizabeth Warren have good reasons to stay to the left on these issues. She's trying to make sure there's no daylight between her and Bernie Sanders. Biden, on the other hand, is more focused on a general election, and he has the flexibility to do that.
STUTZMAN: Biden, because he benefits from being, at this point, the grandfather, almost the standard-bearer in this race, he's got the headroom to stake out these much more mainstream positions that would put him in probably really good shape against Donald Trump.
LIASSON: That is, if Biden ends up being the nominee. So far, the former vice president's lead has been very durable. But that hasn't quieted the worries of many Democrats who consider him a fragile front-runner. Tonight Biden gets another chance to help doubting Democrats imagine him on a debate stage against Donald Trump.
There is one challenge Biden, Warren and Sanders probably don't have to worry about tonight, and that's a generational challenge. There are plenty of younger candidates in the race; seven of them will be onstage in Houston. Although they've tried, so far none of them have been able to dislodge the top three Democrats, all of whom are in their 70s.
Mara Liasson, NPR News, Washington.
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