President Trump reassured three GOP Senate incumbents this week that he would back their reelection bids against any primary challengers.

Politico reported Thursday that Trump called Sens. John Barrasso John Anthony BarrassoMurkowski: Supreme Court nominee should not be taken up before election Battle lines drawn on precedent in Supreme Court fight Sunday shows - Ruth Bader Ginsburg's death dominates MORE (R-Wyo.), Deb Fischer Debra (Deb) Strobel FischerHillicon Valley: Election officials prepare for new Russian interference battle | 'Markeyverse' of online fans helps take down a Kennedy | GOP senators unveil bill to update tech liability protections Google, Apple, eBay to meet virtually with lawmakers for tech group's annual fly-in Congress botched the CFPB's leadership — here's how to fix it MORE (R-Neb.) and Roger Wicker Roger Frederick WickerHillicon Valley: DOJ indicts Chinese, Malaysian hackers accused of targeting over 100 organizations | GOP senators raise concerns over Oracle-TikTok deal | QAnon awareness jumps in new poll Shakespeare Theatre Company goes virtual for 'Will on the Hill...or Won't They?' Republican Senators raise concerns over Oracle-TikTok deal MORE (R-Miss.) to tell them he would support them in the 2018 midterm elections and that he hopes they win.

"The president has been very supportive, and Senator Barrasso is grateful that President Trump would take the time to call him directly to express his support," Barrasso aide Dan Kunsman told Politico.

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The president is expected to eventually offer formal endorsements of the incumbents, according to the report.

That Trump is seeking to reassure Republicans facing potentially primary challenges from insurgent candidates comes as his former chief strategist, Stephen Bannon, prepares to launch an electoral assault on establishment Republicans.

The move also aligns Trump more closely with Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellOcasio-Cortez to voters: Tell McConnell 'he is playing with fire' with Ginsburg's seat McConnell locks down key GOP votes in Supreme Court fight Video shows NYC subway station renamed after Ruth Bader Ginsburg MORE (R-Ky.), who has warned that the anti-establishment candidates backed by Bannon, now the executive chair of Breitbart News, are unlikely to win in general elections.

According to Politico, Bannon has privately expressed a desire to oust Barrasso, Fischer and Wicker, all of whom have received McConnell's support.

Two months after leaving the White House, Bannon has already become a thorn in the side of McConnell and the GOP establishment. The former chief strategist backed Roy Moore in Alabama's Senate GOP primary race. Moore defeated incumbent Sen. Luther Strange Luther Johnson StrangeSessions hits back at Trump days ahead of Alabama Senate runoff The biggest political upsets of the decade State 'certificate of need' laws need to go MORE (R-Ala.) in a runoff last month, despite Strange receiving endorsements from Trump and McConnell.

Bannon has openly declared a "season of war" against McConnell and the establishment wing of the Republican Party and is looking for candidates to challenge GOP incumbents in 2018.