BILLINGS, Mont. (AP) — Montana U.S. Sen. Jon Tester has built a huge fundraising lead over four Republicans hoping to challenge him this fall — and he may need it as outside money starts pouring into a race the GOP wants to pick up to maintain its majority.

Tester, a two-term Democrat from Big Sandy, raised $2 million in the first three months of 2018 and had $6.8 million in cash on hand as of March 31, according to campaign filings with the Federal Election Commission.

That's five times more cash than all four major Republican contenders combined: Big Sky businessman Troy Downing, former District Judge Russ Fagg, state Sen. Al Olszewski and State Auditor Matt Rosendale.

Fagg reported the most cash remaining, $633,000, among the Republicans in the June 5 primary. Rosendale was close behind with $541,000 in cash.

Rosendale's getting a strong push from political committees bankrolled by conservative mega-donor Richard Uihlein ("YU-line") of Illinois. Those two committees — Restoration PAC and Americas PAC, have spent more than $1.2 million to support Rosendale. Three more so-called super PACs spent an additional $84,000 on his behalf.

No other Republican in the race has benefited from independent expenditures, according to FEC records.

Restoration PAC spokesman Dan Curry said the group was committed to electing "conservative public officials who believe in a strong and fiscally-sound America." He declined to say how Rosendale better fit that description than his primary opponents, who have also touted their conservative values.

"We're not disparaging any of the other candidates," Curry said. "We feel that Montana is a key pickup opportunity for Republicans and we're committed to electing Mr. Rosendale to the Senate."

Downing's campaign has brought in the most money to date among the Republicans — $1.3 million, including $1 million of his own money. Making his first run for office, Downing has been burning through money quickly as he seeks to raise his name recognition and had less than $100,000 remaining as of March 31.

Olszewski reported $48,000 in cash remaining.

Republicans have been aiming to topple Tester ever since his extremely-narrow victory over former Sen. Conrad Burns in 2006. Those hopes were buoyed when Republican President Donald Trump won Montana by more than 20 percentage points in 2016 over Democrat Hillary Clinton.

Tester has brought in more than $11.4 million total this election cycle. He enjoyed a similar fundraising lead early in the 2012 election before outside groups spent more than $10 million opposing him. He won by four points that year over former Rep. Denny Rehberg.