A new CBS News-YouGov Battleground poll of Democrats in early states finds Biden more vulnerable than many previously supposed. Of the four earliest contests, Biden is comfortably ahead only in South Carolina, where he gets 43 percent of the vote. In the other three states he’s in a statistical tie with Sen. Bernie Sanders (I-Vt.) or Sen. Elizabeth Warren (D-Mass.), and sometimes both of them:

Iowa: Biden 29 percent, Sanders 26 percent, Warren 17 percent

New Hampshire: Warren 27 percent, Biden 26 percent, Sanders 25 percent

Nevada: Sanders 29 percent, Biden 27 percent, Warren 18 percent

Sen. Kamala D. Harris (D-Calif.) and South Bend, Ind., Mayor Pete Buttigieg register in the mid-single digits in each state, and no other candidate gets more than 3 percent. Overall, in those four states plus the 14 more holding contests through Super Tuesday, 26 percent of voters support Warren, 25 percent Biden, 19 percent Sanders, 8 percent Harris, 6 percent Buttigieg, and everyone else registers at 4 percent or less.

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You might think that Biden’s troubles would come from Sanders and Warren now being seen as equally electable. Yet on that front he remains the clear leader, with 77 percent of Democrats saying he would likely win against Trump, compared with only 58 percent for Sanders and 55 percent for Warren. According to CBS, though, “the movement toward Warren is not attrition from Biden, but of lower-tier candidates losing supporters who have gone to Warren or, in lesser numbers, to Sanders.” With about 30 percent of respondents still supporting other candidates, the path is now clear for Warren or Sanders to establish herself or himself as the front-runner without having to chip away at Biden’s electability argument.

Why the change? Perhaps it’s because when asked whether Democrats’ message should be “return the country to the way it was before Donald Trump took office” or “advance a more progressive agenda than the country had under Barack Obama,” 60 percent of Democrats chose the latter. Most voters get that “the way things were” isn’t good enough, even if many establishment Democrats don’t. That’s bad news for Biden, whose pitch aside from electability can be summed up as “I was Obama’s vice president.”

If Biden’s support does start to fall, both Sanders and Warren are well-positioned to pick up voters. Demographically speaking, Biden’s voters and donors overlap most with Sanders’s voters. But as the Hill’s Krystal Ball and others have pointed out, some moderates have already gravitated toward Warren as their preferred candidate; don’t be surprised if that hedging continues.

For the rest of the candidates in the race, alarm bells should be ringing. As Nate Silver put it on ABC’s “This Week” Sunday, it may be too soon to say there are only three viable candidates in the race, but between Biden, Warren, Sanders, Harris and Buttigieg there’s a “Big Four-and-a-half.” I noted in June that, even this early, history shows candidates need to be pulling in more than a couple percent in Iowa or nationally, and preferably both, by this time to have a chance at the nomination. If other candidates don’t break through after Thursday’s debate, they’re likely already out of time.

Regardless, both this poll and other recent polls have shown that progressives clearly have the momentum in the Democratic race. That’s no surprise: On climate, health care, foreign policy and a host of other issues, the big problems facing the United States demand big solutions. Warren and especially Sanders are the only two candidates offering real change; no wonder more and more Democrats are moving their way.