Risch would replace retiring Republican Sen. Larry Craig, whose arrest in a Minneapolis airport bathroom sex sting captured national headlines last year.
With 36 percent of precincts reporting, Risch led LaRocco by 20 points, 56 percent to 36 percent, CNN reported.
Tuesday's election marked the third time Risch has run against LaRocco. Risch won their race for a seat in the state Legislature in 1982, and two years ago, he beat LaRocco again in their race for lieutenant governor.
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