 Some longtime aides and advisors to President Biden have become increasingly convinced that he will have to step aside from the campaign. And in recent days, they have been trying to come up with ways to persuade him that he should, according to three people briefed on the matter. A small group of Mr. Biden's advisors in the administration and the campaign, at least two of whom have told allies that they do not believe he should keep trying to run for a second term, have said they would have to convince the president of several things.