There's no clear path for Romney to lock up the nomination before the convention

Even if Mitt somehow won every delegate in every coming contest, he still wouldn’t clinch the nomination until Oregon’s primary on May 15.

And if Romney musters only 40 percent of the proportional delegates going forward—equivalent to his share of the popular vote total to date—it would mean the first Republican race undecided when the convention opened in a generation…

Ron Paul’s supporters have been assured that he’ll take their case all the way to the convention—this is a cause more than a candidacy. Newt Gingrich at this point seems motivated as much by personal animus toward Romney as any hope for being the actual nominee. In his speech last night, he repeatedly slammed the former Massachusetts governor as “the Wall Street candidate.” And only Santorum would be susceptible to the traditional bait—get out now and you’ll have a future in the party, perhaps even the next in line for the nomination. But the old-school olive branch—offering Rick the VP slot—seems unlikely in the extreme. There is just too much bad blood…

But none of the three “non-Romneys” seems likely to yield to party pressure. This year is different because the rules of the game have changed. Unlike in years past, more than 40 states are allocating their delegates proportionally, instead of winner take all. That makes it much easier—and much more tempting—for a candidate to stay in the race and drag it out. Candidates this cycle can even offer a high-minded rationale: given the base’s lingering distaste for the longtime de facto nominee Romney, they can argue that such a stand would not be merely self-serving. Moreover, the rise of super PACs in the wake of Citizens United means the party is much less able to choke off funds as a means of pressuring candidates to withdraw.