ANALYSIS/OPINION:

Vermont Sen. Bernard Sanders‘ pledge to stay in the presidential race until the Democratic National Convention in July could be detrimental to Hillary Clinton’s quest for the White House.

We’re already seeing it in the national polls. Mrs. Clinton’s lead over Republican rival Donald Trump is withering in recent surveys — and in some cases is nonexistent. In the five most recent polls, as compiled by Real Clear Politics, the spread between Mr. Trump and Mrs. Clinton has narrowed to plus 0.2 percentage points for Mr. Trump. Before May, most polls had Mrs. Clinton trouncing Mr. Trump by double-digits.

The narrowing results are primary-driven because Mr. Trump has been able to consolidate the GOP base. Republicans who once backed Ohio Gov. John Kasich or Texas Sen. Ted Cruz are now on board with Mr. Trump’s campaign, pledging to defeat Mrs. Clinton in the fall.

According to a New York Times poll, eight in 10 Republicans want the Republican Party to unite under Mr. Trump, even if some still have reservations about his policy. The National Rifle Association’s endorsement of Mr. Trump last week is an example the base coalescing around their national front-runner in preparations for November.

With Mr. Sanders still in the Democratic race, Mrs. Clinton doesn’t have that luxury. According to NBC/Wall Street Journal poll, only 66 percent of Mr. Sanders‘ supporters right now say they’d back Mrs. Clinton over Mr. Trump this fall.

The longer Mr. Sanders waits to endorse Mrs. Clinton, the harder it will be to bridge this divide.

Mr. Sanders has made no indication he’ll get out of the contest come the last primaries in June, repeatedly saying he’s going to take his campaign to the convention. And there’s no reason to disbelieve him. This weekend, Mr. Sanders upped his defiance of the Democratic Party by endorsing the progressive primary rival of DNC Chairwoman Debbie Wasserman Schultz.

It’s been reported that four petitions have been filed in Philadelphia for demonstrations during the Democratic Convention in July, and all four are by Sanders supporters. Mr. Sanders, who has an improbable path to the Democratic nomination, is instead is looking to have an outsized role at the convention by both lobbying to change the “rigged” Democratic nominating process and pushing the party’s platform further left.

If Mr. Sanders turns the Democratic convention into a socialist paradise, that may not only turn off many moderate Democrats, but it also will keep the Democratic base splintered, especially among younger voters who favor Mr. Sanders and have avowed to vote for nobody but him come November.

In the NBC/Wall Street Journal poll, Mr. Trump leads Mrs. Clinton among whites, seniors, men and independents. If the Democratic Party moves increasingly to the left, there’s a good chance Mr. Trump’s numbers among whites, seniors and men will tick up, possibly putting him in the White House.

“In the general election polls since Indiana, we’re seeing more Republican voters rally behind Trump, but Clinton hasn’t gotten a post-nomination bounce yet,” pollster Nate Silver observed on his blog FiveThirtyEight. “So the polls right now are sort of a test of what might happen if the Sanders voters don’t rally behind Clinton. And the answer is that it makes it a closer election — Clinton’s still ahead, but it’s close enough that if something goes wrong (a recession, for instance), you could have President Trump.”