U.S. Senate Republicans will gather behind closed doors on Wednesday to pick their next leader in the face of a public pressure campaign by supporters of President-elect Donald Trump who favor the long-shot bid of a hardline Trump loyalist.

Who senators choose, and whether Trump ultimately endorses a candidate in the final hours, could set the tone for Trump’s attempts to assert control over the legislative branch.

but most already have their eyes on a different election set to take place Wednesday to determine the new party leader in the chamber. Three senators are running to replace Mitch McConnell, who has served atop the Senate Republican Conference for a record ...