WASHINGTON — A defiant President Joe Biden vowed Wednesday to keep running for re-election, rejecting growing pressure from Democrats to withdraw after a disastrous debate performance raised questions about his readiness to keep campaigning, much less win in November.

But increasingly ominous signs were mounting for the president. Two Democratic lawmakers have called on Biden to exit the race while a leading ally publicly suggested how the party might choose someone else. And senior aides said they believed he might only have a matter of days to show he was up to the challenge before anxiety in the party boils over.