Just shy of a month ago, after the first votes were cast in Iowa and New Hampshire, it seemed that the Republican Party faced a fluid and fractious nomination contest, while the Democrats faced a clear-cut choice between two not particularly adversarial candidates. What a difference a few weeks can make.

Now it appears that John McCain is on an unobstructed flight path to the nomination, facing a few crosswinds but no serious navigation hazards, while the two leading Democrats, Barack Obama, and Hillary Clinton, are on the collision course, with the winner taking on serious and possibly disabling damage. And this in a year when the standard metrics — the job performance rating of the president, judgments about the trajectory of the economy, trends in party identification — have seemed overwhelmingly favorable to the Democrats.

How did this happen? Some will give credit to providence, which saw to it that Senator McCain — whose candidacy seemed terminal last July 1 — was able to duplicate, with lesser percentages, his 2000 victory in New Hampshire, then survive a defeat in his best 2000 state, Michigan, then squeeze out a 33% to 30% victory over Mike Huckabee in South Carolina and a 36% to 31% victory over Mitt Romney in Florida.

None of which would have been possible without a collapse in Rudy Giuliani’s support, which was as widely unpredicted as his earlier rise to the top of the polls. Or without the collapse of the candidacy of 2000 McCain supporter Fred Thompson, who led in polls as a non-candidate but lost the lead before he officially declared.

Even so, Mr. McCain now seems a prohibitive favorite for the Republican nomination. He leads in just about all the polls in the big states that vote on Super Tuesday, February 5. Mr. Giuliani has bowed out, and Governor Huckabee’s election night speech reiterated his respect for Senator McCain. Mr. Romney alone has the potential to buy enough ads and possibly derail Mr. McCain this week. But big-time buys did not win for him in Iowa, New Hampshire, or Florida.

In his victory speech, Mr. McCain was at pains to pay respect not only to his rivals, but to the concerns of his critics in conservative journals and talk radio. To his undisputed asset as the longtime and persistent advocate of the surge, which has produced such success in Iraq, he added a stern but seldom-before-voiced resolve to appoint judges who would interpret rather than make law. He was paying — for once, and for the time being anyway — heed to his critics at National Review and his boosters at the Weekly Standard. Memo to Rush Limbaugh: You have been heard.

And what were the Democrats up to when the Republicans were receiving the coordinates of a clear flight path? Heading straight toward each other. The Clinton campaign, defeated in Iowa and nearly in New Hampshire, scraping by in Nevada, and expecting a clobbering in South Carolina, faced a choice between losing clean and winning ugly. What is amusing is that so many liberal commentators were surprised when the Clinton apparat, with the unhesitation of a shark, chose the latter option.

Bill Clinton and other Hillary Clinton surrogates got busy playing the race card against Barack Obama. They belittled his victory in South Carolina and profited from her victory in Florida — Democratic Party rules forbade candidates to compete — where elderly women, Latinos, and Jews, all heavily pro-Clinton (or anti-Obama) constituencies, are heavily represented.

As they will be, to varying extents, in the Super Tuesday states, especially California. Extrapolating from all but one of the Florida results, Senator Clinton will be the big winner there. The exception: Floridians making up their minds at the last minute were evenly split between Senators Obama and Clinton.

Disgust over the Clintons’ tactics has been a staple of liberal magazines and blogs and evidently inspired Edward Kennedy’s endorsement of Mr. Obama. Political history mavens will recall that Mr. Kennedy, well after he had lost the 1980 nomination to Jimmy Carter, continued his campaign and bitter denunciations all the way to the national convention.

Will he urge such a course on Mr. Obama? Hillary Clinton may be content to win ugly. But she may find, as President Carter did, that a crash on the runway is not an appealing spectacle.