Presidential campaigns follow a rhythm. The candidates are generally known by late spring. They are nominated at conventions over the summer. The fall campaign kicks off on Labor Day, at which point the public’s attention turns to the candidates who have spent months preparing for the next eight weeks.

Not this time.

The sudden elevation of Kamala Harris to replace President Biden at the top of the Democratic ticket has transformed what had been a long slog between Mr. Biden and former President Donald J. Trump into a 100-day sprint to Election Day. This campaign is now playing out in fast forward, with a vice-presidential pick, convention, the debate on debates, the production of television advertisements and the crafting of strategy, all taking place in the crunch of weeks rather than months.

Voters will begin casting ballots in Pennsylvania, one of the critical battleground states, as soon as mid-September.

Analysts from both parties said that timeline is likely to benefit Ms. Harris. Her campaign is looking to ride a burst of momentum, hoping to coast past some of the scrutiny and detailed policy debates that candidates usually experience on the path to the nomination — and leaving Mr. Trump casting about to adjust to a very different opponent.

“A shorter campaign gives significant advantage to Harris,” said Matthew Dowd, who was the chief campaign strategist for President George W. Bush in 2004. He noted that Republicans had spent years “relentlessly attacking” Mr. Biden, and before him, Hillary Clinton, so that “by the time the Labor Day comes, the nominee is solidly defined.”

“Not so with Harris,” he said.

The shortened campaign timeline has already forced the Trump campaign to discard months of preparation and recalibrate for a far different opponent, an energetic 59-year-old Black and South Asian woman whose candidacy has clearly stirred interest from a once dispirited Democratic base.