Democratic Sen. Jon Tester Jonathan (Jon) TesterPence seeks to boost Daines in critical Montana Senate race This World Suicide Prevention Day, let's recommit to protecting the lives of our veterans Filibuster fight looms if Democrats retake Senate MORE (Mont.) is getting an unusual ally as he takes fire from President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE months before a midterm election: A Republican senator.

Sen. Johnny Isakson Johnny IsaksonLoeffler paints herself as 'more conservative than Attila the Hun' in new campaign ad Georgia GOP Senate candidates cite abortion in pushing Ginsburg replacement Loeffler: Trump 'has every right' to fill Ginsburg vacancy before election MORE (R-Ga.) is breaking with Trump and his scathing criticism of Tester, who the president has blamed for sinking Ronny Jackson’s nomination to be secretary of the Department of Veterans Affairs.

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Isakson offered his latest defense of Tester — who faces reelection this fall in a state Trump won in 2016 — at an event in Georgia. He noted that reporting this week appeared to verify some of the allegations against Jackson.

Isakson, the chairman of the Senate Veterans' Affairs Committee, pointed to a CNN story that detailed concerns Vice President Pence’s physician had about Jackson, saying it made Trump’s assertion that the allegations against Jackson were off-base a “false statement.”

“Part of the allegations made in one of the affidavits was verified by [CNN],” he said. “I did my job and every senator has the responsibility, if they’re presented with accusations, to try and seek the truth. And that exonerates everybody who seeks the truth.”

CNN reported that Pence’s physician privately raised his concerns with the White House about Jackson last year. The vice president’s doctor questioned whether Jackson had violated privacy protections for Pence’s wife.

Isakson’s comments come as Trump and his allies have unleashed a rhetorical firestorm against Tester, warning that his decision to go public with the accusations against Jackson make him newly vulnerable going into his November election.

The National Republican Senatorial Committee (NRSC) went up with a statewide digital ad on Wednesday, highlighting Trump’s frustration with Tester and accusing the Democratic senator of leading a “reckless crusade” to block the president’s nominee.

“These disgraceful tactics are low even by Washington’s standards, and have made clear that Sen. Tester has fully embraced life in the swamp. It's time for him to go,” said Calvin Moore, a spokesman for the GOP Senate campaign arm.

America First Action, a pro-Trump super PAC, also went after Tester in an ad, saying he helped “spread false information” about Jackson, who subsequently withdrew his nomination.

During a rally in Michigan over the weekend, Trump called Tester’s actions a “disgrace” and suggested that he knew information that could ruin Tester’s political career.

“Tester started throwing out things that he’s heard. Well, I know things about Tester that I could say, too,” Trump said. “And if I said them, he’d never be elected again.”

National Republicans are trying to seize on Tester’s decision to speak publicly about the Jackson allegations, arguing it underscores that he’s out of touch with the red state where Trump won by more than 20 points.

Though Tester is one of several red- and purple-state Democrats up for reelection, he’s largely been able to fly under the radar amid other high-profile Senate battles.

He’s been helped by a bitter, four-way fight in the GOP primary between Montana Auditor Matt Rosendale, businessman Troy Downing, former Judge Russell Fagg and state Sen. Al Olszewski.

Marc Short, the White House director of legislative affairs, warned that Trump could take the fight to Tester’s home turf, telling a Montana radio station that staff was trying to find time for him to make a trip.

But any doubling-down by the White House could be undercut on Capitol Hill, where senators and staffers note that Isakson and Tester have worked to keep the Senate Veterans' Affairs Committee bipartisan.

Sen. Jerry Moran Gerald (Jerry) MoranLobbying world This World Suicide Prevention Day, let's recommit to protecting the lives of our veterans Hillicon Valley: Zuckerberg acknowledges failure to take down Kenosha military group despite warnings | Election officials push back against concerns over mail-in voting, drop boxes MORE (R-Kan.) told reporters that while he wished Jackson had gotten a hearing, he had “no complaints” about Tester’s actions.

“My impression is that Chairman Isakson and Sen. Tester were having regular and constant communications. ... I’m not second-guessing Sen. Tester. He apparently believed they were accurate, truthful and needed to be said,” he told reporters late last week.

The Daily Beast reported on Tuesday that the White House is frustrated with Isakson, accusing the senior GOP lawmaker of not doing enough to warn them about the accusations against Jackson.

But a spokesperson for Isakson pushed back, saying that Isakson’s “first call” after learning about the accusations on April 20 — days before they appeared in the press — was to White House chief of staff John Kelly John Francis KellyMORE.

“Senator Isakson informed General Kelly of the new information brought to the committee and asked for guidance from the White House. He was told that day to proceed with the nomination,” the spokesperson said.

Isakson’s staff was also made aware of the accusations, sat in on the interviews of some of the witnesses and was kept in the loop throughout the process. Tester and Isakson, underscoring their coordination, sent a joint letter to the White House asking for more information on Jackson.

Isakson, who has defended the committee’s handling of Jackson’s nomination, said on Monday that he is “not in the critiquing business” and “every senator has the right to exercise their options.”

Tester raised eyebrows after he did a round of interviews giving the first detailed public accounts of the allegations against Jackson, which included improper dispensing of prescription drugs and creating a toxic work environment. Jackson has denied the charges.

The spokesperson for Isakson added the office received a “heads up” from Tester’s office before he gave the interview and was broadly aware that he was going to discuss the allegations. They also received a copy of the memo before Tester’s office released it publicly.

A Senate Democratic campaign aide pointed to Isakson’s comments as a sign that the White House’s strategy to try to politicize the fight over Jackson’s failed nomination is “backfiring.”

“The fact that the Republican chairman on the committee is endorsing Senator Tester’s approach speaks for itself,” the aide said. “And [it] is just further confirmation of Senator Tester's long and well-documented record of fighting for veterans.”

Nathaniel Weixel contributed to this story.