DENVER — As Americans poured into airport terminals, public squares and congressional town hall-style meetings over the past month to vent their anger over President Trump’s policies, Debbie Dooley started hearing from fellow conservatives.

“Phone call after phone call from people: ‘Why aren’t we out supporting President Trump?’” said Ms. Dooley, a Tea Party activist in Georgia.

So on Monday, conservatives around the country got out the Make America Great Again hats, unfurled American flags and held rallies at state capitols and on courthouse steps, seeking to rekindle the populist fervor that helped vault Mr. Trump to the White House and stick up for a president whose approval ratings have taken a beating during five storm-tossed weeks in office.

But the modest crowds that showed up for the noontime rallies from New Jersey to Georgia to Colorado offered a reminder of what liberal groups learned eight years ago in the face of Tea Party anger at President Barack Obama: Stoking grass-roots energy is often easier for the outraged resistance than for the party pulling the levers in Washington.