Belgrade, Montana (CNN) Air Force One is landing in places this fall it rarely has before. And it's never flown so often to Big Sky Country.

President Donald Trump on Saturday made his fourth trip to Montana since July, which political historians here believe is more than any other US president. Harry Truman visited three times, but over a four-year span.

Trump is more invested in Montana's Senate contest than almost any other, telling friends that his sweetest vengeance of the election season would be knocking off Tester.

This race has become Trump vs. Tester -- at least in the President's eyes -- with Republican candidate Matt Rosendale far more of a bystander.

"How the hell did you elect this guy?" Trump thundered during a visit to Missoula two weeks ago.

Tester sounds far less eager to pick a fight and is trying to keep the race focused on the differences between himself and Rosendale.

"The President coming is a good thing," Tester told CNN. "I would like to see him get around and not just do rallies, but actually see some of the challenges we have in a rural state like Montana. I mean, he's from New York City, he could learn a lot."

As the battle for control of Congress rolled into a frenetic final weekend, with the President flying from Indiana to Montana to Florida on Saturday alone, he is telling allies he is all but resigned to Republicans losing their grip on the House. He warned supporters on Friday in West Virginia, saying: "It could happen."

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But it's the fight for the Senate -- and the prospect of picking up this Montana seat -- that's driving Trump in the waning hours of his first midterm election campaign.

While a handful of red state Democrats could fall on Tuesday, no other Senate contest has become so single-handedly competitive, because of Trump's own involvement. Sens. Heidi Heitkamp of North Dakota, Claire McCaskill of Missouri and Joe Donnelly of Indiana were always facing steep challenges, while Tester was seen as the senator most likely to glide through a difficult Democratic year.

Until he agitated Trump.

The President said Saturday he has not forgiven Tester for raising questions that ultimately led to t he embarrassing withdrawal of Dr. Ronny Jackson, Trump's personal physician, as the nominee for secretary of veterans affairs.

"He might be a nice guy, except he tried to destroy Adm. Ronny Jackson, who is the most clean-cut, wonderful person that you ever met. He tried to destroy him," Trump said. "And I've never forgotten it. And it's honestly one of the reasons I've been here so much."

Now, the race is the ultimate test for Trump.

It will determine if he is the exception to the rule for how a president's popularity often doesn't translate to other candidates in a midterm election. And it will offer a window into whether his loyal supporters will follow his lead against Tester, a third-generation farmer well known to friends and foe alike who has managed to win two Senate terms as a Democrat in a deep-red Republican state.

In an interview, Tester sought to downplay the Trump influence.

"This is a tough race. We knew it was going to be a tough race two years ago, regardless of who came out of the Republican primary," Tester said after rallying supporters in Livingston. He added, "But the fact of the matter is this is a race between myself and Matt Rosendale, and the differences are huge."

Heading into the final weekend, strategists on both sides told CNN they believe the race is excruciatingly tight. Republicans hope the President's visit Saturday will inject a jolt of electricity into Rosendale's candidacy by awakening any Trump supporters who aren't paying close attention.

While Trump has put far more skin in the game than other presidents have in midterm campaigns, crisscrossing the country with one rally after another, he still worries his supporters aren't following the election closely because his name isn't on the ballot.

"You hear midterms and you go sleep, right?" Trump lamented to supporters on a recent night in Missouri, acknowledging he had rarely paid attention to midterm races before he jumped into politics three years ago.

Even though Republicans approach Election Day with a narrow advantage in holding on to control of the Senate, their majority is almost certain to remain slim, with every vote critical, particularly if Democrats capture the House and reshape the political order in Washington.

Trump bluntly acknowledges the Montana seat is more personal than firewall.

"It's not that we need the vote so badly," he told supporters late last month in Missoula. "I can never forget what Jon Tester did."

As the top Democrat on the Veterans' Affairs Committee, Tester raised pointed and persistent questions about the professional conduct of Jackson. The President ultimately withdrew Jackson's nomination to lead the Department of Veterans Affairs, but has made clear that he hasn't moved on.

When Trump arrived for his first visit in July, Tester placed an ad in more than a dozen newspapers across the state: "Welcome to Montana and thank you President Trump for supporting Jon's legislation to help veterans."

Since then, the atmosphere has become far more acrimonious.

The outcome of the Tester-Rosendale race will test whether an appetite still exists among the electorate for a bipartisan check-and-balance on Trump. Two years ago, he defeated Hillary Clinton here by 20 points even as voters re-elected Gov. Steve Bullock, a Democrat.

For Tester to win, he needs Trump voters.

During visits to Great Falls in July, Billings in September, Missoula in October and just outside Bozeman on Saturday, Trump has repeatedly tried to nationalize the race and turn Tester, the farmer from Big Sandy, into a puppet of Sen. Chuck Schumer of New York, the Senate Democratic leader.

"He will never vote for us," Trump said Saturday, devoting a significant portion of his speech to blasting Tester as a "far-left Democrat."

For his part, Tester is doing everything he can to keep the race focused on local issues like access to health care and public lands. One of his closing ads of the campaign features an old meat grinder where he lost three fingers in a childhood accident.

"I was 9 years old when I lost my fingers in this meat grinder," Tester says in the ad. "My parents paid for the hospital because our health care didn't cover anything."

Rosendale, a former Maryland developer who moved to Montana in 2002, served three terms in the state Legislature before becoming state auditor. He has clung tightly to Trump, often wearing his signature blue suit and red tie.

The race could also be influenced by a late development: Libertarian candidate Rick Breckenridge threw his support behind Rosendale this week.

His name will still be on the ballot, but Breckenridge said he favors Rosendale, which angered some Libertarian activists in the state. Six years ago, the Libertarian candidate captured more than 6% of the vote in a race Tester won by only 4 percentage points.

While strategists on both sides believe that the minds of most voters are already made up -- most Montanans, in fact, cast their ballots by mail and about two-thirds of the electorate already has -- Trump believes he can increase Republican turnout with another visit and one final attempt at branding Tester as a Democrat.

"The Democrats have truly turned into an angry mob," Trump said on his last trip here. "And your senator is one of them."

Yet it remains an open question whether the President can paint Tester as a run-of-the-mill Democrat or particularly angry. The senator was all smiles on Friday as he fired up audiences, reminding them of his strong tie to the land.

"What we really need to talk about," he said, "is Montana."