Before this year, the only time Mr. Tester had shared a ballot with a presidential race was in 2012, when President Barack Obama coasted to a second term. Mr. Obama lost Montana by 13.5 points that year, but Mr. Tester won his race by four points.
In Mr. Tester’s 2018 re-election, he defeated Mr. Rosendale, 50.3 percent to 46.8 percent. That loss factored into the decision from Republican leaders, including Senator Steve Daines of Montana, to recruit Mr. Sheehy into this year’s race. Mr. Daines oversees the party’s Senate races as chairman of the National Republican Senatorial Committee.
Mr. Daines has, in several ways, helped Mr. Sheehy win support from Trump loyalists and deep-pocketed Republican donors, two forces inside the party that have regularly worked at cross-purposes in recent years. Mr. Sheehy has been endorsed by Representative Marjorie Taylor Greene of Georgia, and a super PAC backing Mr. Sheehy has collected millions from wealthy Wall Street executives.
Mr. Sheehy has also contributed about $1 million to his own campaign, which spent more than $4 million last year and entered this year with about $1.3 million on hand.
But while Mr. Sheehy is seeking his first elected office, Mr. Rosendale is a well-known figure in Montana Republican politics. The Senate race will be Mr. Rosendale’s eighth political campaign in the past 14 years. In his previous seven contests — four federal races, two state legislative campaigns and one for state auditor — Mr. Rosendale won five and lost two.