Nearly 100 federal judges nominated by Presidents Barack Obama and Bill Clinton — and even Jimmy Carter — would be eligible to take semiretirement by the time the next president is inaugurated, a status that allows them to continue to preside over cases but creates an official vacancy that could be filled by a presumably younger full-time replacement.

Anticipating that at least some of those long-serving judges named by Democrats would step aside once a president more to their ideological liking took office, liberal judicial activists have begun a new effort to recommend possible successors who could immediately be funneled into the judicial pipeline. Those successors would not shift the ideological balance of the courts, but like Mr. Trump’s young conservatives, they would have staying power.

“It is essential to be ready on Day 1 of a new administration with names to fill every vacancy,” said Nan Aron, the president of the Alliance for Justice, the 40-year-old liberal judicial advocacy group. “This is to start identifying people so the new president won’t waste a minute in addressing this need.”