The longest pre-season in presidential campaign history is finally over. Iowa’s caucuses are just four months away, but because of the customary no-A-politicking zones around Thanksgiving and Christmas, the candidates will really have closer to 90 days of full-scale rhetorical battle before the voting begins.

Labor Day has customarily served as the traditional kickoff of the campaign season — for general elections. This year, an earlier-than-ever schedule of primaries and caucuses makes Labor Day an obvious starting point for the final charge toward the nominations.

While the calendar is still in flux, Iowa is likely to hold its first-in-the-nation caucuses the first week of January, two weeks earlier than ever before. State officials are even struggling to resist the possible need to move the caucuses to late December as more and more states leapfrog toward Iowa’s opening position.

The unprecedented pace after Iowa gives candidates precious little time to maneuver. The bulk of delegates to the nominating conventions will be chosen within a month — mainly on Feb. 5, when there will be caucuses or primaries in about 20 states, from New York to California.

The compressed calendar makes the next four months as mind-bogglingly complicated as they are critical for the candidates, even though most have been campaigning and fundraising for nearly a year. The unusually long pre-season provides clues to what’s next, but voters have a way of surprising the experts once they actually get into the mix.

Although they face talented and well-financed opponents who are positioned to topple them, it’s clear that Sen. Hillary Rodham Clinton of New York won the Democratic pre-season and former Mayor Rudy Giuliani of New York bested the Republican field. Giving Clinton and Giuliani the pre-season nod is based on their consistent leads in national polls as well as their strong showings in early-voting states. Both have campaigned remarkably well as front-runners, demonstrating the instincts and fortitude it takes to stay on top.

It is stunning that Giuliani has so far been able to sell his party’s dominant conservatives on the notion that his other credentials should justify overlooking his moderate-to-liberal views on abortion, gay rights and other hot-button social issues. Much of his success with this strategy has been focused on relentlessly attacking Democrats — especially Clinton — as out-of-sync liberals who can’t be trusted with the nation’s budget or security.

Clinton’s jujitsu on the Iraq War is almost as remarkable as Giuliani’s co-opting of conservatives. She has deftly moved away from her early support for the war while never caving to pressure for a complete disavowal of her vote to authorize it. Despite still voicing support for keeping troops in Iraq indefinitely, Clinton appears to be suffering no meaningful backlash from the anti-war crowd. Indeed, among Iowa Democrats, who are dominated by passionate war opponents, she has even been moving up in recent polls.

In persuading voters to consider overlooking what would be their major political weaknesses, Clinton and Giuliani effectively survived the pre-season. But that could change as voters now begin to pay much closer attention. And there are plenty of candidates who could overtake the front-runners.

Fred Thompson will present Giuliani with his first big test this week, when the former senator from Tennessee formally enters the race. But Thompson might have sapped the enthusiasm of his own supporters by waiting so long. Former Gov. Mitt Romney of Massachusetts looms as Giuliani’s biggest threat if Thompson fades. Already it appears that the former mayor’s camp is prepared to accept Romney as the Iowa favorite. Giuliani skipped last month’s GOP straw poll there because he expected Romney to win — and he did, with ease. Former Gov. Mike Huckabee of Arkansas is a good bet for most promising dark horse in the GOP race. A surprisingly strong second-place finish in Iowa’s straw poll and an affable campaign personality have kept him afloat.

Sen. Barack Obama of Illinois and former Sen. John Edwards of North Carolina are the only Democrats truly nipping at Clinton’s heels. Among the dark horses, Gov. Bill Richardson of New Mexico is doing well. The trouble for Obama and Edwards is that they are splitting the anti-Clinton sentiment among Democrats, and they appear likely to keep doing so once the voting starts. At this juncture, Clinton could lose the nomination only if one of her closest rivals clearly eclipses the other in the coming months.

For the sake of excitement, hopefully the voters will completely scramble the field, pushing other candidates to the forefront. Iowa voters especially seem to enjoy proving the experts wrong. And they are famous for delaying their final choices until the last month before voting. If that happens again, then the candidates have only a couple of months for hard campaigning before the die is cast.

Contributing Editor Craig Crawford is a news analyst for NBC, MSNBC and CNBC. He can be reached at ccrawford@cq.com.