HELENA -- Jon Tester, a populist farmer, won a decisive victory over John Morrison in the Democratic primary for the U.S. Senate on Tuesday and will face three-term Republican incumbent Sen. Conrad Burns, who won his primary handily, in November.

Tester, outspent by Morrison by nearly a 2-to-1 margin, led nearly from the start of the vote counting Tuesday night. He ended up defeating Morrison, the state auditor, by about 25 percentage points in the five-way primary.

Although Tester was the decided underdog in the five-candidate Democratic Senate race, he gained great momentum in closing weeks of the campaign through an extensive grass-roots effort.

Burns, meanwhile, coasted to an easy victory over state Senate Minority Leader Bob Keenan to capture the four-way Republican primary. Burns topped Keenan by a better than a 3-to-1 margin.

Montana's Senate race is among the most watched nationally because Burns is rated as one of the most vulnerable Republican senators because of his link to convicted lobbyist Jack Abramoff.

Here were the latest unofficial results, with 86 percent of the votes counted:

Democrats:

--Tester, 54,989 votes or 60.3 percent.

--Morrison, 32,984 votes or 36.2 percent.

--Paul Richards, 1,273 or 1.4 percent.

--Robert Candee, 1,231 votes or 1.4 percent.

--Kenneth Marcure, 698 votes or 0.8 percent.

Republicans:

--Burns, 60,691 votes or 71.6 percent.

--Keenan, 19,367 votes or 22.9 percent.

--Bob Kelleher, 3,626 votes or 4.3 percent.

--Daniel Lloyd Neste Huffman, 1,045 votes or 1.2 percent.

Over the clamor of a victory party in a Missoula hotel, Tester said in a telephone interview. "I thought we had a good chance of winning this thing. The numbers surprised me.''

Tester attributed his win to his campaign's grass-roots efforts involving hundreds of volunteers. Morrison outspent him $1.14 million to $662,805.

"We hit the ground running tomorrow, and it's another campaign for five months,'' Tester said. "It's overdrive now. It's for real. It's all out. We did a lot of hard work getting here and there's no looking back.''

Tester had argued that he was the only Democrat who could go "belly to belly and toe to toe'' with Burns on the ethics issue.

That was Tester's code language for suggesting Morrison couldn't match up with Burns. The reason was that Morrison, before his election as auditor, had had an extramarital affair with a woman in Bigfork. That woman later married a Flathead Valley businessman, David Tacke, who was investigated by Morrison's office for securities fraud. Some critics said Morrison's office went soft on Tacke, a charge Morrison adamantly denied.

Morrison said he called Tester around 10:30 p.m. and conceded the race. He told his supporters in a Helena restaurant and bar that "deciding to run for the United States Senate was the biggest decision of my career and one of the biggest decisions of my family's life.'' The effort, he said, is bigger than just one person or one race.

"Which is why tonight our journey does not end, but rather it begins,'' he said.

Morrison said he ran to "restore integrity'' to Montana's U.S. Senate seat once occupied by Mike Mansfield. He said there is "much more'' to be said of Burns' involvement in an ongoing lobbying scandal involving confessed felon and former lobbyist Jack Abramoff.

"The general election will be about a senator who sold out the people of Montana,'' Morrison said.

After being declared the winner Tuesday night and listening to a concession call from Morrison, Tester called Morrison "a stand-up guy'' and said he was pleased with the auditor's pledge to unite behind him in the fall.

Burns, in an interview Tuesday night, said he was pleased by his margin of victory. He said he doesn't believe in election referendums but said, "It's about our party and what we stand for.''

The senator said he was forced to mount an aggressive primary race, spending $3.6 million, because he's been under attack since last summer by the Montana Democratic Party and the Democratic Senate candidates, who have criticized his ties to Abramoff.

Burns said Montanans "have not heard anything but bad stuff'' about him, so he had to fight back aggressively.

Burns said Tester is a good campaigner and, like he does, has a voting record that "I think will be contrasted to mine.''