Senator Jon Tester, a Democrat, is up for re-election in Montana. Republicans, knowing they might lose some of their own Senate seats, are eager to flip his — and in a state that went for President Trump by 20 percentage points, they very well could. Matt Rosendale, the state auditor and a Maryland native with an accent to match, won the Republican primary.

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Meanwhile, Democrats are hotly pursuing Montana’s only House seat, currently held by Greg Gianforte. But in the first quarter of 2018, Mr. Gianforte raised more money than all his Democratic challengers combined.