Bernie Sanders endorsed Hillary Clinton for president on Tuesday, but a lot of people have argued that Sanders squandered political capital by waiting so long after the final primaries to get behind the presumptive nominee. Indeed, Clinton’s favorability among Sanders’s primary voters is rising, and the vast majority of Sanders’s voters are already backing her. But the same polling also shows a smaller bloc of Sanders supporters still reluctant to get behind Clinton. Sanders’s endorsement, in other words, might still be helpful to Clinton.

Roughly 1 in 5 Sanders supporters say they are going to vote for a third-party candidate. Gary Johnson and Jill Stein combined for 22 percent of the vote among Sanders’s supporters in a recent NBC/WSJ survey and 21 percent in a recent Suffolk University survey. Johnson won 17 percent of Sanders backers in a Pew Research Center poll (the poll did not test Stein). The average third-party support among Sanders’s voters in the three surveys, 20 percent, is significantly higher than the 13 percent of all voters who say they’d back Johnson or Stein. (Younger voters, who voted for Sanders in overwhelming numbers in the primary, are also far more likely to say they’d choose a third-party option or “someone else,” according to these surveys and a new poll from the University of Chicago with the Associated Press-NORC Center for Public Affairs Research.)

Most of the #FeelTheBerners who now say they’ll go third party would otherwise be backing Clinton:

WITH THIRD PARTY WITHOUT THIRD PARTY POLLSTER CLINTON TRUMP CLINTON TRUMP DIFF. IN CLINTON’S MARGIN NBC/WSJ 63% 9% 78% 10% +14 Pew Research Center 67 6 85 9 +15 Suffolk University 63 7 73 9 +8 Average 64 7 79 9 +12 Sanders supporters more likely to back Clinton without third-party candidates All percentages are rounded.

Whereas Clinton beats Trump by an average of 57 percentage points among Sanders’s supporters when third-party candidates are mentioned, she wins by 69 percentage points when these voters are forced to choose between Clinton and Trump. This may not seem like too big of a deal given that Clinton is winning overwhelmingly in both scenarios. Perhaps, it won’t be. But given that there were over 13 million people who voted for Sanders in the primary and the 12 percentage point swing mentioned above, Clinton could net somewhere in the neighborhood of 1.6 million votes if the Sanders supporters currently backing third-party options all voted for either Clinton or Trump.

Of course, Sanders’s supporters are not going to vote for Clinton just because Sanders tells them to. But we’re talking about a very specific group of Sanders’s voters who have already shown a willingness to choose Clinton over Trump. I’m not sure that there’s a better person to aid in persuading them to vote for Clinton than Sanders himself. If Sanders throws his full weight behind Clinton’s candidacy combined with the tendency, at least historically, of voters abandoning third-party candidates as Election Day approaches, it’s not difficult to imagine Clinton could net well over a million votes. If the election ends up being close, Sanders’s help to Clinton could be the difference between her winning and losing.