He won the White House with just 46 percent of the vote. He consistently polls below 50 percent — and has struggled to broaden his coalition.

Another possible complication: Rep. Justin Amash, a Michigan independent who left the Republican Party last year, is considering running for president. Even if he had a marginal effect on the results, the presence of a third-party candidate with his background might be enough to affect the outcome — much the same as the Green Party’s Jill Stein is thought to have affected the results in several key states in 2016.

Against that backdrop, whether Trump wins or loses in November, it's unlikely that he will capture a majority of the popular vote.

“With an approval rating in the low- to mid-40s, barring any unforeseen developments — which in this world, if there’s anything there are unforeseen developments — the expectation is a majority would be a reach for him in the popular vote,” said Lee Miringoff, director of the Marist College Institute for Public Opinion.

If Trump is going to “thread the needle” as he did in 2016, Miringoff said, “any division to create just a wrinkle in the data, a crease in the data, is desirable for him … So he’s playing the cards he’s dealt, and that is, he’s not likely to get a majority, try to keep the Democratic enthusiasm down and get Biden into the mix, and hopefully don’t do it in a way that arouses Democratic sentiment any more than it already needs to be.”

The president’s tweets and public statements are just one aspect of the effort to foment Democratic unrest. His campaign ran ads on Facebook inviting Sanders supporters “feeling berned” to instead “join the winning team.” GOP allies have long been pitching narratives about divisions within the Democratic Party to left-leaning media outlets.

“We do that stuff constantly,” said one Trump ally involved in the effort, describing a bid to “get behind enemy lines” to sow distrust among Democrats in Biden and the Democratic National Committee.

Most Republicans do not expect a significant number of Sanders’ supporters to vote for Trump in the fall. But Trump’s allies have been encouraged by Biden’s difficulties during the primary with younger voters, and they believe they can win over some Sanders supporters and depress others’ enthusiasm for the nominee.

While the vast majority of Sanders supporters say they will back Biden in November, a Morning Consult poll last week showed 7 percent of them planned to vote instead for Trump.

“Understandably, there are a lot of Bernie Sanders supporters who feel disaffected after the Democrat establishment shoved Bernie aside for the second time,” said Tim Murtaugh, a spokesman for Trump’s reelection effort. “There are also a lot of them who were attracted to Bernie’s outsider message. We say to them, Donald Trump is the ultimate disrupter and they can find a home on his side.”

The divide-and-conquer rhetoric is distinct from Trump’s messages for his broader re-election audience, in which he has depicted Biden and Sanders as cut from the same leftist cloth.

Striking at Biden for his support for undocumented immigrants, tax and climate change proposals, Trump’s campaign is running digital ads referring to Biden a “big government liberal” and concluding “he’s not so different from radical socialist Bernie Sanders.”

When Sanders endorsed Biden on Monday, Brad Parscale, Trump’s campaign manager, said in a prepared statement that “this is further proof that even though Bernie Sanders won’t be on the ballot in November, his issues will be. Biden had to adopt most of Bernie’s agenda to be successful in the Democrat primaries. One thing that is missing is enthusiasm, however, as almost no one is excited about a Biden candidacy.”

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Tom Perez, the chairman of the Democratic National Committee, said Monday that Democrats are “really well poised to come to come together as a party,” and Biden has taken steps to appeal to progressives since clinching the nomination, including issuing proposals to expand Medicare and forgive some student debt.

Kate Bedingfield, Biden’s communications director, said Tuesday that Trump is reacting to “the sweeping victories and overwhelming turnout for Joe Biden during the greatest comeback in American political history,” as well as “the incredible endorsements from Senator Sanders and President Obama.”

“This is all coming from a place of desperation, but he can't hold back the tide,” she said. “He’s not hiding the ball on his belief that the more people who turn out to vote, the less likely he is to be re-elected.”

On Wednesday, another leading progressive and Democratic campaign rival, Sen. Elizabeth Warren, also endorsed Biden.

Jeff Cohen, co-founder of the pro-Sanders online activist group RootsAction.org, said Biden still has work to do to “forge some unity” in the Democratic Party, with a critical decision looming with his choice for vice president.

But he predicted Trump’s efforts will fall flat.

“That he thinks he can sow these divisions on the Democratic Party side,” Cohen said. “Frankly, the more he opens his mouth, the more he unifies the Democratic Party … That’s the irony here. He is so hated by both wings of the party that when he does this, you know … it backfires.”

Alex Isenstadt and Zach Montellaro contributed to this report.