Dem
51
GOP
49
Usually, a national convention helps the person nominated there. This year might be different. The Republican National Convention could actually help Joe Biden more than it helps Donald Trump. Trump will be preaching to the choir and is likely to throw lots of red meat to the base. This won't get him any new voters and might alienate moderate Republicans watching. If, however, he moderates himself—for example, by emphasizing that abortion should be up to the states, that could infuriate some abortion hard liners who decide to stay home, to "teach the Republicans a lesson." Progressives try to "teach the Democrats a lesson" all the time. Two can play this game.
But whatever, the national spotlight will be on Trump and his veep all week. It won't be on Democrats telling Biden to drop out. Those stories will be pushed far into the background. It may be hard to restart them in 2 weeks, with the Democratic National Convention so close.
When Trump announces his plans for a second term, Democrats will have an easy target, explaining why those plans would be a disaster for the country. That would be too good an opportunity to miss. It would be political malpractice for Democrats to be attacking Biden as weak when Trump was busy saying things they abhor and possibly touting a running mate Trumpier than Trump, if it is Sen. J.D. Vance (R-OH). All their fire next week will be aimed at Trump and Biden could join in, making him look strong rather than weak. Going back to attacking him a week later would be very difficult. If Biden survives until the week after the Republican convention, he's probably safe.
That said, Biden is definitely risking his legacy by being so stubborn. If he were to drop out now, no matter who was nominated and who won, his legacy would be: "He put the country above his personal interest. A true patriot." If he stays in and Trump wins, a lot of backseat drivers (and history book writers) are going to be saying: "The signs were so clear. He was behind in all the polls. He botched the debate. He made a half-baked attempt to recover. Much of his party wanted him out. Donations dried up. How could he have been so blind?" It's a huge gamble. (V)