Senator Charles E. Grassley entered a packed town hall in southeastern Iowa on Tuesday with a list and a plan.

The last time he met with constituents, Mr. Grassley told Iowans who filled every seat in a small meeting room, the session had devolved into a heated gripe session about President Trump’s policies. So this time, in an effort to keep things civil, Mr. Grassley, a 91-year-old Republican, had brought a written list of topics he wanted to talk about before opening the floor.

“Has anybody come to talk about the farm bill?” Mr. Grassley asked hopefully, looking up from the list in his hands.

They had not.

Instead, for the next hour, Mr. Grassley was bombarded with complaints about Mr. Trump’s tariffs, anger about his actions on immigration, grievances over his efforts to take an ax to the federal government and frustration that the Republican-led Congress had done little to rein him in.

“We would like to know what you, as the people, the Congress who are supposed to rein in this dictator — what are you going to do about it?” one man asked, gesticulating as much of the crowd applauded. After criticizing the Trump administration’s deportations, the man scowled, leaned toward Mr. Grassley and asked, “Why won’t you do your job, Senator?”

The senator opened his mouth, but his tentative reply was drowned out by others echoing their disapproval.