Senate Majority Leader Mitch McConnell has vowed not to confirm a successor to the late Supreme Court Justice Antonin Scalia, but some Republicans worry that President Obama could install a replacement anyway.

"We're in recess, so Obama can, in theory, appoint someone," a senior GOP aide told the Washington Examiner. "He could appoint [Vice President Joe] Biden tonight if he wanted to."

Obama wouldn't be the first president to use a recess appointment to place a justice on the high court. There are have been 12 justices appointed to the Supreme Court while the Senate was in recess. Dwight Eisenhower was the last president to use that power — he did so three times, according to Scotusblog.

In one case, critics speculated that Eisenhower made his pick to boost his re-election chances that year.

"It will be said, of course, that President Eisenhower chose Justice Brennan because he is a Democrat and a Roman Catholic," the Chicago Tribune wrote at the time. "Our guess is that the appointment will help a little to hold votes of Democrats who like Ike and will strengthen the president's earned reputation as a man quite free of religious prejudice, but if the appointment had been intended primarily for electioneering purposes, the chances are that Mr. Eisenhower would have selected someone with a larger reputation as a Democrat and a Catholic."

Obama used the recess power aggressively in the past. In January of 2012, he appointed three people to the National Labor Relations Board when the Senate was not officially in recess due to pro forma sessions. In 2014, the Supreme Court ruled 9-0 in NLRB v. Noel Canning that those appointments are unconstitutional because the Senate, by it's own rules, was not in session at the time the appointments were made.

But that's not the case now. The Senate is scheduled to be in recess until Feb. 22, giving Obama nine days to mull the idea. If Obama were to pursue that route, Republicans likely would argue that the Senate was out of session for too short a time for Obama to take advantage of the recess appointment power. But the Supreme Court has never said exactly how long a recess must be for the president to make recess appointments.

"It's a live threat," the aide said.