WASHINGTON — President Obama is rethinking his plans to withdraw from the political arena after he leaves office next year, hinting to friends and supporters that he wants to add his voice to the shellshocked Democratic activists and elected officials who are now angrily vowing to oppose Donald J. Trump’s presidency.

White House aides say they expect the president to try to refrain from criticism during the transition because of his belief in the importance of a courteous and dignified transfer of power. But while the president holds out hope that he might influence Mr. Trump, he has made it clear that once out of office he will not remain silent if Mr. Trump goes too far in undoing his legacy.

“I’m going to be constrained in what I do with all of you until I am again a private citizen,” Mr. Obama, who will be living a few miles from the White House next year, told a meeting this past week of Organizing for Action, the group that maintains his political movement. “But that’s not so far off.”

Dozens of liberal advocacy groups, which have received a flood of donations and new members in the chaotic days since Mr. Trump’s defeat of Hillary Clinton, are gearing up for years of clashes with Mr. Trump. After eight years of advocacy on behalf of the Obama agenda, they are racing to recast themselves as bulwarks against Mr. Trump’s expected assault on an array of Democratic policies.