Sen. Mark Kirk of Illinois on Friday became the first Republican senator to call for an up-or-down vote on Supreme Court nominee Merrick Garland, saying on a Chicago radio show that his colleagues ought to “just man up and cast a vote.”

That Kirk would be first to break with Senate Majority Leader Mitch McConnell (R-Ky.) and other GOP colleagues, who believe the next president should pick the replacement for Justice Antonin Scalia, is not particularly surprising: Kirk was already one of two Republican senators, with Susan Collins of Maine, to call for hearings.

Kirk faces what is perhaps the most difficult Senate reelection race in the nation — running during a presidential election year as a Republican in a state that hasn’t voted for a Republican president since George H.W. Bush in 1988. He is pitted against Rep. Tammy Duckworth (D), who has already sought to tie Kirk to national Republicans, including GOP presidential front-runner Donald Trump....