Montana voters will decide the challengers to Democratic U.S. Sen. Jon Tester and Republican U.S. Rep. Greg Gianforte in Tuesday's primary election. Voters also will be picking candidates to run in the general election for 38 contested legislative races, two Public Service Commission primaries and two district judge races. A look at the top races:

U.S. SENATE

An influx of outside money has entered the four-way Republican contest to challenge incumbent U.S. Sen. Jon Tester, a seat the GOP has long coveted and sees as winnable after President Donald Trump's wide margin of victory in Montana in 2016.

State Auditor Matt Rosendale has benefited from about $3.2 million spent as of Monday by outside groups boosting his candidacy and attacking his opponents.

Rosendale, a former state legislator who unsuccessfully ran for U.S. House in 2014, says he's the only one capable of beating Tester.

Retired state judge Russ Fagg of Billings has mounted an attack on Rosendale, the perceived front-runner, while playing up his own Montana roots. Fagg pledged to be tough on Mexican cartels that bring methamphetamine into the state.

Businessman Troy Downing of Big Sky says his experience as an Afghanistan combat veteran distinguishes him from the field. He's pumped $1 million of his own money into the race, but has been dogged by pending criminal charges accusing him of buying Montana resident hunting and fishing licenses when he was still a California resident. He contends the charges stem from a conspiracy against him by state wildlife officials.

State Sen. Al Olszewski has trailed in campaign donations. The Kalispell lawmaker says that makes him less beholden to the special interest groups that have backed his opponents.

U.S. HOUSE

Republicans have held Montana's only U.S. House seat for more than two decades, but the five Democrats running for their party's nomination say this is their party's year.

The crowded field of political newcomers — and one former legislator — wants to harness the political activism that has welled up since the 2016 election across the nation and in Montana through marches and protests. They say Republican Rep. Greg Gianforte is more vulnerable than past incumbents after he won a special election last year to fill Interior Secretary Ryan Zinke's unexpired term.

Donors have favored two Democratic candidates who are making their first run for office. One, consumer protection attorney John Heenan of Billings, has a populist message that he says mirrors some of the themes Trump campaigned on, but hasn't followed through with: getting rid of corporate cronyism, improving health care and building infrastructure.

The other, former land trust leader Grant Kier of Missoula, also seeks to appeal to voters across party lines, saying many Republicans and independents feel abandoned in the Trump era. He said he would sit down with anyone who has good ideas and look to bring dignity and respect back to political discourse.

Former state legislator Kathleen Williams of Bozeman calls herself a progressive who is also business-friendly. She would be the first woman to hold Montana's congressional seat since Jeannette Rankin left office in 1943.

Rounding out the field are first-time candidates Jared Pettinato, a former U.S. Department of Justice attorney, and John Meyer, an environmental attorney