WASHINGTON—Republican senators’ secret ballot vote Wednesday for a new leader will test how much lawmakers want to shake things up as they enter an era defined by the populist energy that propelled President-elect Donald Trump back to the White House.

House and Senate Republicans will huddle this week to select a new slate of leaders for the next year’s Congress, the first step towards legislating in the looming Trump administration. The moves come as the GOP is on the brink of securing a trifecta in Washington: Trump is headed to the White House,

John Thune of South Dakota, John Cornyn of Texas and Rick Scott of Florida are running in a secret ballot election Wednesday to lead the GOP conference and replace longtime GOP leader Mitch McConnell,