WASHINGTON — When it came to filling a Supreme Court vacancy during the 2016 presidential election year, Senator Mitch McConnell had a constant refrain: Let the people decide. But should a high court seat become open in 2020, Mr. McConnell has already decided himself.

“Oh, we’d fill it,” Mr. McConnell, the Kentucky Republican and majority leader, gleefully told a friendly Chamber of Commerce audience back home in Paducah on Tuesday.

Mr. McConnell regularly celebrates his history-altering 2016 decision to thwart President Barack Obama from filling a vacancy that occurred with 11 months remaining in his term, saying the seat should be kept open until a new president could be elected and inaugurated. But he has been laying the groundwork to change course ever since Donald J. Trump was elected president. Tuesday’s remarks were only his most definitive: He would not be bound by the standard he himself set in preventing Judge Merrick B. Garland from being seated on the high court.

The comments immediately drew howls of blatant hypocrisy from Democrats and progressive allies. They said it underscored their view that Mr. McConnell was unprincipled and acted out of purely partisan motives in 2016 when he single-handedly decided to blockade Mr. Obama’s choice to replace Antonin Scalia after the court icon’s death that February.