1. He has pulled Romney to the right on key issues. Romney's challenge has always been to convince the right wing of his party he is conservative enough without moving too far to the right and endangering his standing with swing voters in the general election. Santorum, a social (if not necessarily small-government) conservative of unimpeachable credentials, has made that much harder. Romney's latest tax plan, for example, closely resembles a conservative plan previously released by Santorum. It's been a similar story on a range of issues. But perhaps the most dangerous topic to have been pulled to the right on is contraception and women's issues, which have become a flashpoint in the race. Santorum's strict views forced the formerly pro-choice Romney to take stronger stands. Even with Santorum fading in recent weeks, those views have stuck to Romney, and he now trails President Obama among women by as much as 19 percent.

2. He's cost Romney vital time and money. The likely result of the race is just what many observers predicted a year ago: a Romney nomination. But it's been a far more difficult battle than anticipated. After tying Santorum in Iowa -- and later, it emerged, losing to him -- Romney was forced to be much more aggressive toward him than anticipated. He spent millions of dollars and three months trying to hold off Santorum (and, to a lesser extent, Gingrich) that would have been more profitably spent attacking Obama. While Obama has built robust organizations in swing states like Ohio and New Hampshire, Romney has had to rely on "swat teams" that have helped him win a series of important primaries but have left behind little of the infrastructure he'll need to compete with the president. And while Romney has recently begun attacking Obama, his coffers have been depleted attacking Santorum.

3. He has exacerbated Romney's weaknesses. Watching from Chicago, the Obama reelection team must have been delighted to see Santorum's attacks against Romney. While he refused to join in when Rick Perry and Gingrich assailed Romney as a "vulture capitalist," Santorum had no qualms refuting Romney's claims that he was a job creator. His campaign gleefully jumped to capitalize on a Romney aide's comment that the general election was like an Etch A Sketch, with the candidate wielding the toy on the stump. Most crucially, Santorum has successfully depicted Romney as close to President Obama on many issues, especially health-care reform. Notably, he called Romney "the worst Republican in the country to put up against Barack Obama." And just last week, he released a brutal ad drawing parallels between Obama and Romney:

In his announcement in Gettysburg, Santorum spoke about the importance of Republican unity to defeat Obama and retake the Senate. But with his quixotic quest to fight Romney, Santorum has ensured that it will be much harder for the GOP to take up residence in the White House in January.