This spring, as the Republican presidential primary race was just beginning, the Democratic National Committee commissioned polling on how the leading Republicans — Donald J. Trump and Ron DeSantis — fared against President Biden in battleground states.
But now, as Mr. Trump’s lead in the primary has grown and hardened, the party has dropped Mr. DeSantis from such hypothetical matchups. And the Biden campaign’s polling on Republican candidates is now directed squarely at Mr. Trump, according to officials familiar with the surveys.
The sharpened focus on Mr. Trump isn’t happening only behind the scenes. Facing waves of polls showing soft support for his re-election among Democrats, Mr. Biden and his advisers signaled this week that they were beginning to turn their full attention to his old rival, seeking to re-energize the party’s base and activate donors ahead of what is expected to be a long and grueling sequel.
On Sunday, after Mr. Trump sought to muddy the waters on his position on abortion, the Biden operation and its surrogates pushed back with uncommon intensity. On Monday, Mr. Biden told donors at a New York fund-raiser that Mr. Trump was out to “destroy” American democracy, in some of his most forceful language so far about the implications of a second Trump term. And on Wednesday, as the president spoke to donors at a Manhattan hotel, he acknowledged in the most explicit way yet that he now expected to be running against “the same fella.”
The mileposts all point to a general election that has, in many ways, already arrived.
David Axelrod, the architect of Barack Obama’s presidential campaigns, said engaging now with Mr. Trump would help Mr. Biden in “getting past this hand-wringing period” about whether the president is the strongest Democratic nominee.
“The whole predicate of Biden’s campaign is that he would be running against Trump,” Mr. Axelrod said. “Their operative theory is, once this is focused on the race between Biden and Trump, that nervousness will fade away into a shared sense of mission. Their mission is in getting to that place quickly and ending this period of doubt.”
Mr. Trump has undertaken a pivot of his own, skipping the Republican debates and seeking to position himself as the inevitable G.O.P. nominee, with allies urging the party to line up behind him even before any primary votes are cast.
Mr. Biden, in his remarks to donors on Monday on Broadway, issued a blunt warning about his likely Republican opponent.
“Donald Trump and his MAGA Republicans are determined to destroy American democracy,” the president said. “And I will always defend, protect and fight for our democracy. That’s why I’m running.”
Mr. Biden is planning to follow up those off-camera remarks with what he has billed as a “major speech” about democracy. The White House said the speech, in the Phoenix area the day after the next Republican debate, would be about “honoring the legacy of Senator John McCain and the work we must do together to strengthen our democracy.”
Instead of attending that debate, on Wednesday, Mr. Trump is making a trip to Michigan planned during the autoworker strike — aiming to appeal to the blue-collar workers who helped deliver him the White House in 2016. The Biden campaign has been building out a plan to counter him there, in addition to its planned response to the Republican debate.
Mr. Biden is facing a moment of turbulence. His son Hunter was just indicted. The Republican-controlled House is moving toward impeachment. Polls show a lack of Democratic excitement for his re-election. And voters continue to dismiss rosy economic indicators and hold a more dour financial outlook, even as the president has tried to sell a success story under the banner of “Bidenomics.”
The focus on democracy and Mr. Trump is not new for Mr. Biden. The opening images of his 2024 campaign kickoff video showed the violence on Jan. 6, 2021, at the Capitol, and he delivered two major addresses on the stakes for democracy before the 2022 midterm elections.
Yet Mr. Biden, White House officials and his campaign have remained studiously silent on the biggest developments surrounding Mr. Trump this year: the 91 felony counts he faces in indictments in four jurisdictions. The president wants to avoid giving credence to the evidence-free idea that he is personally responsible for Mr. Trump’s legal travails.
“Trump was his own worst enemy throughout the last year,” said Anna Greenberg, a Democratic pollster and strategist. “While most of the punditry talked about how much the indictments helped with his base, it hurt with everyone else.”
Ms. Greenberg said it was almost inevitable that Mr. Trump would energize Democratic voters if he won the Republican nomination again. “For better or worse, Trump has been the driver of the highest turnout we’ve seen in the last 100 years in the last three election cycles,” she said. “I fully believe Trump will be a driver of turnout in 2024 as well.”
“Joe Biden is an unmitigated disaster and his policies have hurt Americans and made this country weaker,” said Steven Cheung, a spokesman for Mr. Trump. “President Trump continues to dominate the primary because voters know he’s the only person who will beat Biden and take back the White House.”
Mr. Biden also faces a key fund-raising deadline at the end of September. In his 2020 run, he struggled to raise money from small donors online — until he became the nominee against Mr. Trump, when he shattered fund-raising records.
Mr. Biden’s fund-raising during the reporting period that ended in June showed that he was again slow to attract vigorous support from small donors online, though people familiar with the campaign’s fund-raising have said the numbers have been better during the current quarter.
At the start of this year, Democrats close to the White House had hoped for a long and bloody Republican primary that would consume the party, leaving its eventual nominee undecided until deeper into 2024 and by then weakened.
But as Mr. Trump has consolidated his lead — he has consistently drawn more than 50 percent support in national polling averages since late spring — Democrats are resigned to something of a political consolation prize: the chance to draw an early contrast with Mr. Trump.
Some of Mr. Biden’s top aides and advisers have believed, despite ample polling earlier in the year that suggested the opposite, that Mr. Trump would be a tougher general-election opponent than Mr. DeSantis or any of the other Republican presidential candidates.
This spring, months before the D.N.C.’s pollsters stopped testing matchups between Mr. Biden and Mr. DeSantis, the party’s polls showed the Florida governor faring better than Mr. Trump against the president in battleground states.
Now, Democrats in the few states where the 2024 presidential election is likely to be decided have come to the same conclusion as Mr. Biden: It’s going to be Mr. Trump again.
“I don’t see any of the other Republicans gaining any traction against Trump,” said Representative Dina Titus of Nevada, a member of the Biden campaign’s national advisory board. “DeSantis has dropped even further in the polls and nobody else has moved much ahead.”
Most of the advertisements Mr. Biden’s campaign has broadcast so far have been positive messages highlighting his record on foreign policy and the economy. But a spot about abortion rights that has run for three weeks shows Mr. Trump boasting that “I’m the one who got rid of Roe v. Wade” and saying, in a quickly recanted 2016 interview, that women should be punished for having abortions. The ad also shows Mr. DeSantis and Senator Tim Scott of South Carolina speaking about legislation to restrict abortion.
Mr. Biden has spoken, off and on, about Mr. Trump for months. He has also used several right-wing figures, including Senators Ron Johnson of Wisconsin and Tommy Tuberville of Alabama and Representative Marjorie Taylor Greene of Georgia, as stand-ins to paint the whole Republican Party as in thrall to Mr. Trump.
In a Labor Day speech in Philadelphia that Mr. Biden’s aides described as framing the forthcoming general-election campaign, he made five references to “the last guy” and one to “my predecessor” but never mentioned Mr. Trump by name.
The shift toward Mr. Trump was reflected in Mr. Biden’s remarks to donors this week. At his New York fund-raiser, Mr. Biden said Mr. Trump’s name four times in 12 minutes.
“I don’t believe America is a dark, negative nation — a nation of carnage driven by anger, fear and revenge,” he said. “Donald Trump does.”
Mr. Biden’s Instagram feed, meanwhile, offers a road map of the issues on which his campaign wants to draw a contrast with Mr. Trump in 2024: abortion, guns, infrastructure, jobs and prescription drug prices.
“I think,” said Representative Chrissy Houlahan of Pennsylvania, “that we are set for a rematch.”