A slew of meetings last week between Breitbart News chairman Steve Bannon and potential GOP primary challengers has left top Republicans scrambling to find a way to thwart the ex-Trump aide's effort to reshape their party, fearing Bannon could cause permanent damage.

Bannon has made one thing clear since leaving the White House in August after spending seven months as President Trump's chief strategist: the political lives of several incumbent Republicans are about to become that much harder.

The fiery media mogul, who returned to the helm of Breitbart News mere hours after departing the West Wing, is said to be working with influential conservative donor Robert Mercer to help insurgent GOP candidates launch serious challenges against establishment-friendly senators in 2018.

"Assuming this happens, and they are credible candidates, and that Mercer generously funds them, this can't be good news for either the incumbent senators or the Republican Party," Larry Sabato, founder and director of the University of Virginia's Center for Politics, told the Washington Examiner.

"Sometimes vigorous competition can strengthen a primary winner, yet more often a bitter primary race divides a party, bankrupts the winner, and emboldens the opposition," Sabato added, listing the myriad problems Republicans could face if Bannon plows ahead with his battle against the GOP establishment.

An unrelenting critic of Senate Majority Leader Mitch McConnell, Bannon has already begun targeting one of the Kentucky Republican's favorite incumbents – Alabama Sen. Luther Strange, who is locked in a tight runoff race against former state Supreme Court justice Roy Moore. Much to the chagrin of Bannon, Trump endorsed Strange last month.

The Bannon-backed Great America Alliance launched a 30-second ad Tuesday, urging Alabama voters not to "let the swamp take over" their state. The ad cast Strange as a "big time lobbyist … in the pocket of Mitch McConnell." Politico first reported the TV spot.

"If [Bannon] considers a guy who's voted with Trump 91 percent of the time ‘part of the establishment,' he's going to find himself waging war against more than half our members," said one GOP aide. "I have a hard time believing the White House will tolerate that."

Several GOP senators have already urged Bannon to focus on helping the GOP chip away at the number of congressional seats Democrats currently hold.

"I wish they would focus on Democrats instead of Republicans," John Cornyn, the Senate's second-highest ranking Republican, told reporters this week.

Cornyn's colleague, South Dakota Sen. John Thune, later added Bannon's endeavor is "not particularly productive" toward the GOP's goal of advancing Trump's agenda by coalescing around certain pieces of legislation and securing a greater congressional majority next fall.

"We ought to stay focused on the task at hand," Thune said.

At the top of Bannon's hit list are two senators up for re-election in 2018 whose records are filled with extensive criticism of Trump: Dean Heller of Nevada, and Jeff Flake of Arizona.

Flake has slammed the president recently for his treatment of young illegal immigrants and his response to the violence in Charlottesville, while Heller has attempted to maneuver his way back into Trump's good graces by backing the failed GOP healthcare bill and working with him on tax reform.

But Heller's evolving posture hasn't stopped Bannon from laying the groundwork to unseat him next November. The political strategist met with perennial candidate Danny Tarkanian last Thursday to discuss his campaign against the incumbent Nevada Republican.

"The result of the conversation was that he and Breitbart were going to be fully behind me in my race," Tarkanian told the Washington Examiner. "We talked a little bit about myself and my campaign, then he expressed some of his positions and talked a lot about the issues he thinks will be prevalent next fall."

Tarkanian said Heller has proved to Nevadans he is a political opportunist who "keeps changing his positions based on what he wants."

Those closest to Heller said the incumbent Republican is confident in his current relationship with Trump and his ability to defeat Tarkanian or any other GOP insurgent.

"As far as the president is concerned, we're working closely with him and he's never shown anything but a desire to continue to develop that relationship," an aide to Heller told the Washington Examiner.

The aide cast Bannon's desire to "challenge senators who already have a fight ahead of them next cycle" as counter-productive, claiming the former West Wing staffer should unite with all Republicans "around getting things done."

"I think Steve Bannon, now that he's left the White House, is trying to find a role for himself," the aide said.

A spokesperson for Flake did not respond to a request for comment.

Heller, Flake, and other GOP lawmakers whose actions have left Bannon seething may face even more trouble if the Breitbart chairman succeeds in sowing frustration among outside groups who support Trump and have grown tired of congressional Republicans criticizing him.

At least one pro-Trump super PAC has already declined to rule out backing Republican primary challengers in the upcoming midterm elections.

"At this time, we are observing the field of GOP candidates and, with the exception of Senator Luther Strange, have not yet thrown our support behind any candidates," said Erin Montgomery, a spokeswoman for America First Action, an outside group that recently added ex-Trump campaign manager Corey Lewandowski to its ranks.

"That could certainly change, of course!" Montgomery told the Washington Examiner.