WASHINGTON — By the summer of 1938, President Franklin D. Roosevelt had had enough. His own party controlled both houses of Congress, yet the latest elements of his New Deal were stalled. Exasperated by fellow Democrats standing in the way, Roosevelt resolved to push some of them out of office.

Nearly 80 years later, President Trump’s former chief strategist, Stephen K. Bannon, has declared a “season of war” to push out problematic Republicans in midterm elections, just as Roosevelt tried to do to balky Democrats. But Roosevelt’s purge backfired. Not only did he fail to take out his targets, but he also emboldened them, all but dooming his domestic program for much of the rest of his presidency.

Whether Mr. Bannon’s purge will be more successful has become the consuming question in Washington these days. Fresh off his victory backing Roy S. Moore over an incumbent Republican senator in a special primary election in Alabama, Mr. Bannon vowed last week to support challengers in 2018 to every Senate Republican except Ted Cruz of Texas. Over the weekend, he took aim at Senator Mitch McConnell of Kentucky, the majority leader, pledging to find someone “to be Brutus to your Julius Caesar.”

While Mr. Bannon acts on his behalf, Mr. Trump is playing coy, much as Roosevelt did at first. Hosting Mr. McConnell at the White House last week, the president sought to publicly patch over differences and said he might rein in Mr. Bannon’s planned assault on Republican incumbents. “Some of the people that he may be looking at, I’m going to see if we talk him out of that, because, frankly, they’re great people,” Mr. Trump said.