When President Trump swept back into office, his dejected opponents watched as his return was greeted not with mass resistance but with a sense of resignation.

Protesters stayed home. Corporations and executives rushed to curry favor. Even some Democrats made overtures to Mr. Trump, as he and his allies boasted that they had popular opinion on their side.

But just over 100 days into his second term, seeds of dissent to Mr. Trump’s agenda, governing style and expansion of executive power have grown in fits and starts across the country. The opposition is sturdier than it once appeared.

Demonstrations have increased in size and frequency. Town halls have become unruly and combative, pushing many Republican lawmakers to avoid facing voters altogether. And collective efforts by universities, nonprofit groups, unions and even some law firms have slowly started to push back against the administration.

“There is a momentum developing,” said Gov. JB Pritzker of Illinois, a Democrat who first ran for office in 2018 because of his revulsion to Mr. Trump’s first term. “Now, I feel like there are people standing up and speaking out and taking up and seeing that this is the right thing to do, that it’s going to get worse before it gets better.”

A national movement has not yet flowered: The opposition lacks a leader, a central message or shared goals beyond a rejection of Mr. Trump. Even as some Democrats become more aggressive, their deeply unpopular party is struggling to articulate a unified line of attack — or much of a strategy at all, apart from hoping the president’s approval ratings continue to fall.