BILLINGS, Mont. (AP) — Two Montana Republicans seeking to challenge incumbent Democrat U.S. Sen. Jon Tester in 2018 have chipped in large amounts of their own money to jumpstart their campaigns, campaign filings show.

Big Sky businessman Troy Downing loaned his campaign $350,000 in two installments in May and September, according to filings with the Federal Election Commission. State Sen. Albert Olszewski of Kalispell loaned his campaign $100,000 in September.

The loans make up most of the candidates' total campaign receipts reported to date: $492,000 for Downing and $168,000 for Olszewski.

State Auditor Matt Rosendale has raised the most from outside sources among the Republicans, with $434,000 in contributions through Sept. 30. After subtracting expenses, Rosendale had $354,000 in cash at the end of the reporting period.

Downing had about $307,000 in cash remaining, and Olszewski had $144,000.

Tester is seeking a third term after winning relatively narrow victories in 2006 and 2012. His seat is considered vulnerable because Montana voters gave Republican Donald Trump a 20-percentage point victory over Hillary Clinton in last year's presidential election.

But the Democrat from Big Sandy for now holds a huge financial advantage: $7.9 million in contributions so far this election cycle, including $1.2 million during the most recent quarter. Tester had $5.4 million remaining on Sept. 30.

A fourth Republican, retired District Court Judge Russell Fagg of Billings, declared his candidacy after the campaign finance reporting deadline had passed and did not submit an FEC report.

But Fagg already has been fundraising and established an exploratory committee while he was still on the bench, prompting criticism from Montana Democrats who accused the Republican of running a secret campaign to avoid rules about donor disclosure.

Fagg spokesman Bowen Greenwood would not say how much money the candidate has taken in. "We'll release the numbers when the time is right," he said.

Republicans James Dean of Havre and Ron Murray of Belgrade also have filed to run, but they did not submit quarterly finance reports.