GOP debate could alter course of nomination fight Wednesday night's nationally televised debate represents the last, best chance to alter the course of the Republican presidential contest ahead of high-stakes primaries in Michigan and Arizona next week. Regardless of whether Mitt Romney or Rick Santorum is the winner next Tuesday, those elections will set a new direction for the campaign and determine whether an up-for-grabs nomination race continues far into the spring or even summer. One thing is clear: It won't end as swiftly as it did four years ago, when John McCain had the GOP nomination effectively locked up by this time. If Romney manages to sweep both primaries -- by no means a given, if the latest polls are to be believed -- the Republican campaign may finally start heading down a familiar route, though a much more prolonged one than in recent years. On the strength of those victories, the former Massachusetts governor would be well-positioned to start pulling slowly away from the rest of the field during a fast-moving round of primaries and caucuses in March. According to his campaign advisors, Romney could put the nomination essentially out of reach by April 24, when voters in five eastern states, including Santorum's Pennsylvania, pick delegates. But that's a best-case scenario now for Romney, who had hoped to be much closer to ending the primary phase of his campaign by this point. Down another path lies an even more unpredictable Republican drama. A Santorum victory in one or both of the Feb. 28 primaries would virtually guarantee a prolonged, and potentially damaging, struggle into early June, when California holds its delegate-rich primary, and could conceivably go all the way to the Republican National Convention in Tampa at the end of August. Here's why: Primary campaigns are momentum contests, particularly in the earlier stages. Ultimately, though, delegate-selection rules come into play, and the Republicans are rapidly approaching that point. Romney's grip on the front-runner's spot remains quite shaky. He currently trails Santorum in national opinion surveys of GOP voters. Romney is still ahead in the very early competition for national convention delegates -- with 73 of the 1,044 needed to lock up the nomination, according to the latest FrontloadingHQ tally. Romney has considerable advantages in organization and money -- he out-raised Santorum by almost 10 to 1 through the end of January, according to the latest Federal Election Commission figures -- and he's leveraging that edge right now in various states that will soon vote. What Romney hasn't done, yet, is secure the hearts of Republican primary voters in anything like the way that he's won the wallets of the party's biggest donors. At the same time, new delegate-selection rules -- modeled on the Democratic system that helped produce the 2008 marathon between Barack Obama and Hillary Rodham Clinton -- make it tougher for anyone to win a majority of the convention delegates until well into the spring. Those realities, and looming GOP contests in a dozen states during the first week of March, are combining to make Tuesday a pivotal moment in the 2012 campaign. After losing four states already (five, counting Missouri's non-binding primary), Romney can ill-afford more setbacks heading into Super Tuesday. Romney aides like to spin the fact that McCain lost 19 states last time, but GOP nominees in the modern era seldom lose more than roughly seven or eight, and it is worth noting that McCain failed to win the presidency. For Romney, a Michigan defeat would be disastrous. Romney was born in Detroit and raised in Bloomfield Hills, an upscale suburb. In 2008, he won the Michigan primary by a comfortable nine-point margin over McCain. Losing the state where his father was a popular governor and where the Romney name is fondly remembered by older voters would cast a dark shadow over his presidential prospects and shatter the notion that he's still the front-runner. A Santorum victory in Michigan would not only wound Romney severely and boost the former senator's nomination chances, it would also give fresh impetus to Gingrich, who is skipping the state. Gingrich is hoping to hang around long enough, and do well enough in the Southern primaries in the first two weeks of March to keep his campaign alive. If he does, the odds of a protracted delegate fight increase, in part because new party rules make it harder for a front-runner to amass large numbers of delegates quickly in a multi-candidate field. It is even possible, under this scenario, for the Republican campaign to go all the way to the convention floor, for the first time since 1976. In Kansas City that August, President Gerald Ford held off a determined challenge from the forces of former California Gov. Ronald Reagan, a classic showdown between the Republican Party establishment and its conservative base that resembles the one that is being fought now. In Michigan, a new statewide Marist/NBC poll released Wednesday showed a statistical dead heat: Romney at 37% and Santorum at 35%, with Ron Paul with 13% and Gingrich at 8% among likely primary voters. Polling in Arizona suggests that Romney may have a little more breathing room there, but not much. With a pair of primary races that tight - in a quicksilver year in which overnight changes in voter attitudes have become almost routine - the importance of the Arizona debate (CNN, 8 p.m. Eastern / 5 p.m. Pacific) is hard to overstate. A strong performance by one of the leading contenders - or a major gaffe - could produce a momentum shift that resets the Republican race. paul.west@latimes.com