Six months into his presidential campaign, Mr. Biden is still delivering uneven performances on the debate stage and on the campaign trail in ways that can undermine his message. He takes circuitous routes to the ends of sentences, if he finishes them at all. He sometimes says the opposite of what he means (“I would eliminate the capital gains tax — I would raise the capital gains tax” he said in this month’s debate). He has mixed up countries, cities and dates, embarked on off-message asides and sometimes he simply cuts himself off.

That choppy speaking style puts Mr. Biden at a disadvantage as his front-runner status erodes and he confronts growing pressure to expand his appeal with voters and donors. He faces intensifying competition for moderate support, a formidable liberal foe in Elizabeth Warren, attacks on his family by Mr. Trump and Republicans, and a troubling cash crunch.

At a time when he most needs to convey confidence and forcefulness, some Democrats say, he is instead getting in his own way.

“He does not do well speaking where he isn’t giving a speech,” said Chris Henning, the Democratic chair in Greene County, Iowa, who caucused for Mr. Biden when he ran for president in 2008. “He’s not good in debates and he comes across like he’s stumbling around, trying to figure out what he’s going to say.”