Partial returns from 37 percent of the state's precincts showed Rehberg easily outdistancing little known farmer Dennis Teske, 76 percent to 24 percent, the Billings Gazette reported. Tester, who is running for a second term, was unopposed.
In a statement, Rehberg accused Tester and President Barack Obama of "pursuing a harmful agenda of higher taxes, record debt, taxpayer-funded bonuses for Wall Street executives and a costly, big-government healthcare law that Montanans don't want."
Tester said the race with Rehberg "is between a farmer who holds himself accountable to Montana and someone who's made some pretty irresponsible decisions during his 12 years in Congress," the Gazette reported.
Rick Hill was leading Corey Stapleton in the Republican gubernatorial primary while state Attorney General Steve Bullock won the Democratic nomination for governor.
Presumptive Republican presidential nominee Mitt Romney was an easy winner in the Montana primary, where Obama was running unopposed on the Democratic side.
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