There could still be a ways to go, however. If the delegates continue to break approximately the same way they've broken thus far, he could pick up the necessary number in the late primary state of Utah, where Romney runs strong due to the Mormon population.
The Associated Press has a good rundown of the possible scenarios, with a clean Romney victory being the most likely outcome. However, there are ways that Santorum could continue to drag out the fight and, depending on your point of view, make things more interesting. This includes a scenario involving so-called "superdelegates" - which you might remember from the Democrat primary in 2008.
1. The likeliest route: Romney pulls off a clean win by the time the last state votes in June.The road ahead to the nomination could still have a few bumps in the road even for the frontrunner. It's very clear, however, that Santorum can still yet play a large role in this race.
Romney's about halfway to the magic number, according to The Associated Press delegate count. If he hits his mathematical mark - or if his only rival within shouting distance, Rick Santorum, drops out - Romney instantly becomes the presumptive nominee and the general election race is on.
2. Flying to the rescue: Superdelegates can speed up the finish.
They were empowered just for this sort of scenario. All members of the Republican National Committee automatically attend the nominating convention, and 117 of them are superdelegates whose state party rules leave them free to vote however they choose.
So far most have stayed on the sidelines while the primary plays out. Romney's big win in Illinois and a growing sense of inevitability may draw more superdelegates to endorse him.
3. A contested convention: Suddenly the long-winded roll call of the states gets interesting.
If Romney fails to snag the necessary delegates, Republicans can buckle up for their most tumultuous convention since Ronald Reagan nearly stole the nomination from President Gerald Ford in 1976.
4. Winning by losing: Santorum grabs the chance he's been waiting for.
This is the scenario Santorum's campaign is pushing to explain how he could still get the nomination after trailing in primary voting.
If no candidate wins a majority in the first round, GOP rules require roll call after roll call until one person emerges with a majority. After the first round, the delegates are mostly free to back whomever they want, so bargaining can begin in earnest.
5. A brokered convention: The powerbrokers toss out the candidates and draft someone new.
It sounds crazy after decades of hermetically sealed conventions. And it's certainly a long shot. It would mean rejecting everyone who's received actual votes from the Republican faithful. But some delegates might see that as a more appealing way to compromise.