Liberal activists are already calling on President Trump to keep any possible Supreme Court vacancy open until after the 2020 election, but Senate Majority Leader Mitch McConnell (R-Ky.) has indicated he would fill a court vacancy next year, even though he blocked former President Obama's nominee for most of 2016 after the death of Justice Antonin Scalia.

McConnell argued at the time that voters should have a chance to weigh in on the balance of the court.

Progressive groups say McConnell should apply that same thinking if there is a vacancy between now and Election Day.