Former President Donald J. Trump will make two appearances today, first addressing the evangelical Faith and Freedom Coalition in Washington, D.C., before holding a campaign rally in Philadelphia. President Biden is at Camp David, the presidential country retreat, for the weekend.

The presidential campaign is entering a crucial phase. Mr. Biden and Mr. Trump will face off in an unusually early debate on CNN on Thursday as polls reflect a tightening race. FiveThirtyEight’s aggregate of national polls show Mr. Biden very slightly ahead of Mr. Trump for the first time since recording began in March, while its election forecast shows the outcome of the race as effectively a coin flip.

The Trump campaign has erased the Biden campaign’s early fund-raising lead: Mr. Trump out-raised Mr. Biden for a second consecutive month in May and entered June with more cash on hand. Megadonors also jumped into the race on both sides, with Timothy Mellon, a reclusive billionaire, giving $50 million to a pro-Trump super PAC and Michael Bloomberg, the former mayor of New York City, donating nearly $20 million to support President Biden’s re-election campaign.

Republicans, conservative news outlets and the Trump campaign have long sought to paint Mr. Biden as frail and incompetent as a result of his age, often with the aid of deceptively edited videos circulated online. Mr. Trump has himself often directed those attacks at Mr. Biden, but ahead of the debate he has now insisted that the president is a dangerous opponent who shouldn’t be underestimated.

“I watched him with Paul Ryan, and he destroyed Paul Ryan,” Mr. Trump said in an interview, referring to the 2012 vice-presidential debate between Mr. Biden and Mr. Ryan, Mitt Romney’s running mate. He added: “So I’m not underestimating him. I’m not underestimating him.”

Mr. Trump’s address at the Faith & Freedom Coalition will be his ninth appearance with the group, going back as far as 2011. When the former president addressed the group last year, he was just one of many Republican presidential candidates vying for the support of the Christian right — including former Vice President Mike Pence, Gov. Ron DeSantis of Florida and former Gov. Nikki Haley of South Carolina.

At the time, Mr. Trump had celebrated his role in ending the constitutional right to abortion by appointing three of the six justices who struck down Roe v. Wade. Now Mr. Trump is the presumptive nominee, and his vanquished rivals are off the main stage — replaced by allies contending to be his running mate, including Representative Elise Stefanik and Tulsi Gabbard, and other Trump allies.