Rick Santorum won eleven out of thirty-one state contests and over three million, or twenty-eight per cent, of the total votes cast. If he had stayed in the race he very likely would have won at least half a dozen more states and hundreds of thousands more votes. But sagging support in his home state of Pennsylvania and the growing mathematical likelihood that he would be unable to prevent Romney from winning a majority of the delegates needed to secure the nomination probably contributed to his decision today to drop out of the race.

Despite success beyond anyone’s expectations, Santorum finished the race in much the same place he started it: as a factional candidate unable to attract a broad spectrum of Republican voters. Santorum was never able to grow his coalition beyond the very conservative, mostly evangelical Christians who care deeply about abortion and other social issues and who are highly suspicious of Mitt Romney’s pro-choice past. As important as this bloc is in the Republican Party, it isn’t enough to win the Presidential nomination. Santorum joins a long list of social-conservative niche candidates—Pat Robertson, Pat Buchanan, and Mike Huckabee—that gave a scare to the establishment front-runner but did little more than that.

As for Santorum’s future, it’s unlikely the he will be a major contender for the next Republican Presidential nomination. The cliché about the Republican Party is that it usually nominates the candidate who is “next in line.” Ronald Reagan narrowly lost the nomination to Gerald Ford in 1976 and was nominated in 1980. When Reagan’s term expired, George H. W. Bush, Reagan’s main opponent in 1980 and then his loyal Vice-President, won the nomination in 1988. Bob Dole, a party stalwart who was Ford’s running mate in 1976 and ran for President in 1980 and 1988, finally won his party’s nomination in 1996. And John McCain, who was George W. Bush’s main challenger in 2000, won the G.O.P. nomination in 2008 after the end of the Bush era. This pattern continues in 2012 now that McCain’s major opponent is the presumptive nominee.

But Santorum’s legacy is much more likely to be similar to that of Robertson, Buchanan, or Huckabee than to Reagan, Bush, Dole, or McCain. He seems to have been a placeholder candidate for Republicans alienated from Romney. His voters were as much anti-Romney as they were pro-Santorum, and that means his success in 2012 was sui generis to the 2012 election. He has shown no ability—or interest—to appeal to voters outside the evangelical vineyards and has spent much of this campaign taking positions that would make him unelectable in a general election. He might return to Fox News, where he was previously a paid analyst. But he’s unlikely to be Romney’s running mate—he would do more harm than good for the ticket—or his party’s future nominee.

Photograph by Gene J. Puskar/AP Photo.