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President Donald Trump called three Republican senators to offer his support should they receive primary challenges, two sources familiar with the calls told NBC News on Thursday.

Republicans Deb Fischer of Nebraska, Roger Wicker of Mississippi and John Barrasso of Wyoming all received calls on Wednesday. All have been reported to be on the list of those expected to be challenged next year by Steve Bannon-backed candidates.

"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's chief of staff, Dan Kunsman, told NBC News. "Beyond that, Senator Barrasso considers their conversations private.”

Related: Bannon’s Playbook: How He’ll Wreck the GOP Establishment

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A source briefed on the conversation with Wicker said the senator was surprised by the roughly 15-minute call but welcomed the president's support. Wicker's office did not immediately respond to a request for comment.

A spokesperson for Fischer confirmed that the senator got a call from Trump but would not discuss the contents.

The White House did not immediately respond to a request for comment.

The calls were first reported by Politico.

Bannon, the executive chairman of Breitbart News and Trump's former chief strategist, declared a "season of war against a GOP establishment" last weekend in a speech at the Values Voter Summit.

Trump backed Republican Sen. Luther Strange of Alabama in September, but Strange was defeated by former judge Roy Moore, a conservative firebrand, in a runoff election. Moore was backed by Bannon.

The calls come two days after Trump met with Senate Republican leader Mitch McConnell of Kentucky and boasted about an "outstanding" relationship. McConnell, whom Bannon has criticized, said Monday that he and Trump "have the same agenda."

McConnell on Monday and Tuesday mentioned fringe Republican Senate candidates who had lost in the past, such as Christine O'Donnell, Todd Akin and Sharron Angle, telling reporters that if they didn't back electable candidates, they could lose key races in 2018.

"What do they all have in common? They're in private life, and a Democrat is in the Senate," McConnell said Tuesday of those candidates who lost. "So our goal is to nominate people who can actually win in November."