Deep-pocketed supporters of Senate Majority Leader Mitch McConnell (Ky.) and other GOP leaders have resolved to fight a protracted battle over the next year for the soul of the party in congressional primaries. “It’s shaping up to be McConnell, the Senate Leadership Fund and the Chamber against Bannon,” said Scott Reed, the senior political strategist for the U.S. Chamber of Commerce. “And we will take that fight.”

But the task will not be easy. Strategists from both sides of the party’s divide say recent focus groups and polling have shown that the frustration within the Republican base has only grown since the 2016 election, stoked by an inability to repeal and replace President Barack Obama’s health-care law. President Trump, meanwhile, has continued to cast his presidency in opposition to the current ways of Washington, which could encourage primary voters to buck the system in a way that endangers House and Senate incumbents.

“Just as in 2008, the election did little to let the air out of the tires,” said Steven Law, the president of the Senate Leadership Fund, a super PAC allied with McConnell that plans to spend heavily on Senate primaries in support of incumbents. “The raw material of the electorate is just increasingly volatile.”