When Donald Trump warns listeners about the number of Supreme Court justices that could be appointed by the next president, he usually says "three or four." That's the number he used in his interview with The Post on Tuesday. On occasion, though, feeling energetic, he says that perhaps as many as five justices could be appointed by either himself or Hillary Clinton.

"Even if you can’t stand Donald Trump, even if you think I’m the worst, you’re going to vote for me. You know why? Judges," he said at a rally in Virginia this week according to The Huffington Post's Igor Bobic. "Can you imagine if Hillary Clinton picks five super-libs?"

This is a strong line of argument for Trump. At least one prominent conservative, Hugh Hewitt, has endorsed Trump on the strength of keeping court appointments in the hands of the Republican party. "If Hillary Clinton wins, the Left gavels in a solid, lasting, almost certainly permanent majority on the Supreme Court," Hewitt wrote in an essay published over the weekend. "Every political issue has a theoretical path to SCOTUS, and only self-imposed judicial restraint has checked the Court's appetite and reach for two centuries." The essay was titled, "It's the Supreme Court, stupid."

Earlier this year, before the death of Antonin Scalia, we reported that the Supreme Court was older than nearly any other on record (in part thanks to increasing life expectancies). Scalia was at the upper end of the group average.

That's one indicator that the next president may get to appoint a larger number of justices. (It seems almost certain that he or she will get to appoint at least one: Scalia's replacement.) Another is to look at how many justices have been appointed in the past.

At first blush, it seems likely that Trump's five-justice estimate is predicated on his (or Clinton's) serving two terms. But that's not necessarily the case.

Abraham Lincoln appointed five justices in his first term. William Howard Taft appointed six. (George Washington appointed seven -- but he was starting from scratch.) John Tyler nominated nine, but only one was confirmed.

Lincoln and Taft are outliers, though. On average, presidents have nominated 2.5 justices and seen 1.8 confirmed in their first terms. In subsequent terms, presidents have nominated 2 on average and had 1.5 confirmed.

This is what makes Trump's argument so potent -- and it's why the refusal by Republicans to seat a replacement for Scalia has proven so useful. There's no way of knowing how many justices the next president will appoint, but it could be all of them. At the very least, Republican voters know there will be one seat to fill.

Polling has consistently shown that people who plan to vote for Trump are more likely to say they're doing so to keep Clinton from the White House than because they want to see Trump there. Waving the spectre of a "lib"-heavy Court is an effective way for Trump to appeal to that impulse.