BILLINGS, Mont. — Candidates for Montana's closely-contested U.S. Senate race face a grim reality as they press toward Tuesday's finish: There's a dwindling pool of voters left after large numbers already cast their ballots before Election Day.

Democratic Sen. Jon Tester is facing another stiff re-election challenge after President Donald Trump made an unprecedented four trips to Montana to stump for Republican challenger Matt Rosendale. Control of the Senate is potentially at stake.

More than 300,000 voters had cast their ballots by Sunday, two days ahead of the Nov. 6 election. That's almost 43 percent of all of the state's registered voters, suggesting turnout will be high and also bolstering expectations among election watchers most voters will cast absentee ballots.

On Sunday, the incumbent Tester was making several campaign stops across western Montana. He had a final rally Monday in the city of Great Falls alongside Gov. Steve Bullock and Democratic U.S. House candidate Kathleen Williams.

Rosendale, the state auditor, was attending church services in the state capital of Helena on Sunday and spending time with his family, said spokesman Shane Scanlon.

Both sides have made pitches to veterans as part of their closing election arguments.

"If you want this country to live up to the promise we made our veterans, to make sure our veterans have health care and education benefits and housing benefits, you've gotta' vote," Tester said during a Sunday rally in the college town of Missoula.

He added that his opponent was a "guy from Maryland" backed by East Coast interests. Rosendale is a real estate developer who moved to Montana from Maryland in 2002.

A day earlier in the city of Bozeman and with the president at his side, Rosendale said Tester in two terms had failed to fix the problems facing veterans and has opposed Trump on tax cuts and his picks for Supreme Court.

"To all of the veterans, you can rest assured that when I'm in the United States Senate, President Trump and I will keep our promises to veterans," Rosendale said.

The Republican senate candidate has been boosted by a forceful White House campaign to unseat Tester.

President Donald Trump has gone after the Democrat since April, when Tester derailed the president's first-choice for Veterans Affairs secretary, Ronny Jackson, with allegations of drunken behavior, overprescribing prescription drugs and fostering a hostile work environment.

Jackson denied the allegations, which have been referred to the Pentagon's Inspector General for investigation.

Continuing the White House blitz for Rosendale, Vice President Mike Pence on Monday is scheduled to appear near the city of Kalispell for the candidate and Republican Rep. Greg Gianforte, who is seeking to return for a full term after taking office in a special election last year.

Despite the last-minute activity, increasingly fewer votes are still up for grabs, according to absentee voting data from the Montana Secretary of State's office.

That doesn't mean the frenzied pace will let up in a race that's drawn more than $60 million from outside groups and in donations. The spending marks record for the state and has meant airwaves saturated with negative political advertisements.

Also driving interest this election season is Williams' attempt to become the first Democrat to win Montana's lone House seat since 1997 and citizens' initiatives to raise tobacco taxes and tighten rules on mining pollution.

"We're looking at a huge turnout already," said Jason Adkins, assistant professor of political science at Montana State University Billings.

He added: "If it's a race that's going to tight, you want as many people to turnout as you can. Especially in Montana, where we don't have strong party identification, you have to get out the vote for everyone."