The longest Republican presidential nomination process in recent memory may have come to an end on Tuesday. Not since Ronald Reagan forced Gerald Ford to defend his incumbency all the way to the convention in 1976 has a GOP ticket been contested so deep into the cycle. States that normally have little impact on the Republican nomination, such as California, had begun to prepare for their long-awaited day in the sun.

Disappointment awaits them again. Rick Santorum withdrew from the race two weeks before a critical contest in his home state of Pennsylvania, the only contest on April 24 that anyone expected the former senator to have even a chance of winning against frontrunner Mitt Romney. Recent polling had been mixed, but the trend appeared to show Keystone State voters moving toward Romney. That posed a big problem for Santorum, whose quixotic shoestring campaign had worked miracles to survive as long as it did. Last week's relatively big loss in Wisconsin — a state in which Santorum's Midwest appeal should have given him a boost, and which looks demographically similar to Pennsylvania — probably was the last straw. At the moment, Santorum has revived his standing within the party with the conservative base, but another loss in his home state — where he was crushed in his 2006 Senate re-election bid — could damage his credibility as a candidate in the future. Without a real path to the nomination in the primaries, why take that risk?

Incredibly, Santorum rose from electoral obscurity to top-tier contender for the nomination.

That takes nothing away from Santorum's amazing return from electoral obscurity to top-tier contender for the nomination. As ABC's Rick Klein commented on Twitter, Santorum won 10 states in this contest, an incredible feat for someone who had very little financial backing until he won a surprise victory in the non-binding Iowa caucuses in January. Santorum's relevance in April may have been the most surprising outcome in a cycle that at times seemed defined by surprises among the Republican contenders — most of them unpleasant.

Santorum's success can be credited in no small part to his surprising strength as a candidate. He outperformed his rivals in retail politicking in Iowa and proved himself tenacious when other, more highly-regarded candidates like Tim Pawlenty and Herman Cain pulled out. Santorum also vigorously espoused his pro-life, pro-family agenda while relating both to the broader economic and fiscal concerns that drove the grassroots in the midterm elections. 

However, the main subtext of Santorum's survival was a significant and continuing level of disaffection with Mitt Romney among the conservative base. Grassroots conservatives rallied behind Santorum as the Not Romney after Newt Gingrich faded in Florida and Nevada. Santorum's withdrawal comes after a series of losses in primaries over the last few weeks; Santorum won seven of the last 24 contests while Romney won 16 of them, with one tie for delegates in the Alaska caucuses. Santorum won only one state in the last eight contests (Louisiana). 

The Wisconsin results showed that Romney may have begun to turn the corner with the party's base. According to the exit polls from last week's primary, Romney won almost all of the demographic categories in play. He carried a majority of Republicans (the Wisconsin primary is open to all registered voters), edged Santorum among "very conservative" voters, and won a plurality of independents. In a key indicator of grassroots response, Romney won a near-majority of Tea Party adherents (49 percent). Romney also led among Catholics, whose votes he had been regularly winning, and Protestants, where Santorum had usually performed stronger. He also won a majority among those who approve of embattled Gov. Scott Walker, which has become a cause celebre among conservative grassroots activists, and not just those in Wisconsin.

Romney may have begun to close the deal with the GOP, but not everyone is conceding that the nomination is his. Gingrich has continued to insist that he will compete for the nomination all the way to Tampa. Instead of acknowledging defeat and joining Santorum in a withdrawal, Gingrich made a plea for Santorum's delegates. "I humbly ask Senator Santorum's supporters to visit to review my conservative record and join us as we bring these values to Tampa," reads a press release from the Gingrich campaign. "We know well that only a conservative can protect life, defend the Constitution, restore jobs and growth and return to a balanced budget."

This comes too little, too late for Gingrich. He no longer has enough funding to continue campaigning. Even if Santorum's delegates all switched to Gingrich, donors certainly won't leap to invest in Gingrich's moribund campaign. With Santorum out of the way, Romney will almost certainly sweep the April 24 contests and add almost all of the 231 delegates in play, which would bring Romney close to 900. California's modified winner-take-all primary on June 5 will give Romney most of its 172 delegates, and he will win a big share of Texas' 155 delegates, even if Gingrich manages to put together a campaign for the May 29 proportional primary. That gets Romney past the 1,144-delegate threshold, even without considering the races Romney will easily win in Indiana, Utah, Oregon, and New Jersey, which have another 164 delegates between them.

The race is over, and Romney will be the nominee. This removes the specter of a brokered convention — always a remote possibility, anyway — and instead shifts focus to Romney's choice of running mate. That will be his next big test for the party's base, and his next big opportunity to persuade them to put their skepticism aside and enthusiastically rally to his banner. If he chooses wisely, the convention in late summer will feature a new unity from Republicans — and a focus on their real goal.