Sen. John Kerry (Mass.) has pulled off one of the great comeback acts of recent political history to emerge as the 2004 Democratic frontrunner and probable presidential nominee. But his trials are just beginning. [IMGCAP(1)]

Having decisively won both the Iowa caucuses and New Hampshire primary, he can now expect to rally support from unions and politicians who formerly backed Rep. Richard Gephardt (Mo.) or hadn’t yet committed themselves.

Former Gephardt aides say they don’t expect him to endorse Kerry — or anyone — before next week’s Missouri primary, although Kerry is appealing to Gephardt to help him all but clinch the nomination.

Almost immediately, Kerry also can expect the media — probably with assists from his Democratic rivals and the GOP — to shine a scrutiny laser on his political career. Although in speeches he claims to have “fought for” various causes in Congress, it’s hard to name a major piece of legislation that bears his name.

Chief Democratic rival Howard Dean has pointedly observed, for instance, that Kerry voted not to fight the 1991 Persian Gulf War after Iraq had invaded Kuwait, but then voted to give President Bush power to fight the 2003 war, which a huge majority of Democrats oppose.

The dossier on Kerry also includes a 1995 proposal to cut intelligence funding by $300 million for the next five years and a 1994 proposal to cut $1 billion from the program that coordinates counterterrorism activities.

The Boston Globe observed last year that in 1984, Kerry said he would cancel the B-1 bomber and the B-2 stealth bomber; the Apache helicopter; the Patriot missile; F-15, F-14 and Harrier jets; and the Aegis air-defense cruiser.

The Globe also reported that he advocated cuts in other systems, including the Abrams tank, Bradley fighting vehicle and Tomahawk missile, all critical to U.S. military success in Afghanistan and Iraq.

And while Kerry legitimately surrounds himself with fellow Vietnam War veterans and protests GOP cuts in veterans’ programs, opponents point out that he never sought an appointment to the Veterans’ Affairs Committee, where he could have had an impact on policy.

All that said, Kerry for now is a big winner and a legitimate claimant to the old Bill Clinton title, “Comeback Kid.” In early January, Kerry stood in third place in New Hampshire polls, 25 points behind the then-dominant Dean.

Kerry backer Rep. Ed Markey (Mass.) says Kerry’s new campaign manager, Mary Beth Cahill, developed a comeback strategy that Markey likened to a “give-and-go” play in basketball.

Instead of devoting his energies to must-win New Hampshire, Kerry shifted time and resources to Iowa, and as his fortunes rose there, they rose in tandem back in New Hampshire.

On the day before the Jan. 19 Iowa caucuses, Kerry had risen to 20 percent in the ARG tracking poll, still trailing Dean by 8 points.

On the day after Kerry’s victory in Iowa — and as TV stations were relentlessly re-running Dean’s disastrous concession speech — Kerry had jumped to lead Dean in New Hampshire, 34 percent to 15 percent. At that point, Dean was in third place behind retired Gen. Wesley Clark.

Dean managed to climb his way back into second by Tuesday night — with 26 percent to Kerry’s 39 percent — by chuckling at his Des Moines meltdown and by inviting his wife, Judy, to help humanize him.

The Dean organization distributed thousands of videotapes of the Deans’ interview with Diane Sawyer, in which Mrs. Dean said she couldn’t recall her husband ever getting angry.

Dean campaign aides said they think Dean’s partial recovery also occurred because his supporters resented the media’s “distorting” his Iowa performance, intended not as a “rant” but as encouragement to youthful supporters.

Nonetheless, according to the National Election Pool’s New Hampshire exit poll, 36 percent of voters said they doubted that Dean has the temperament to be an effective president.

Among the top four New Hampshire finishers, Dean had the lowest favorability rating — 59 percent, compared to 71 percent for Kerry — and the highest unfavorables, 37 percent.

Kerry also has benefited from the decline in importance of Dean’s signature issue, the Iraq war, which New Hampshire voters listed third among their concerns.

Sixty-five percent of primary voters said they disapproved of the war, but Kerry out-polled Dean among this group, 37 percent to 35 percent. Among the 28 percent who supported the war, Kerry beat Dean, 36 percent to 14 percent.

Kerry also profited from his opposition to full repeal of President Bush’s tax cuts. Forty-seven percent of voters said repeal should be confined to wealthy Americans. Of these, 39 percent supported Kerry to 25 percent for Dean.

Dean and Kerry nearly evenly split the primary vote among self-identified Democrats, who made up 49 percent of the electorate. But Kerry handily carried the nearly half identifying themselves as independent.

Looking to next week’s primaries, Dean aides suggested that he plans to concentrate on winning in New Mexico and Arizona. Clark vowed to fight there and in Oklahoma, South Carolina, Missouri and North Dakota.

Sen. John Edwards (N.C.), whose supposed “surge” in New Hampshire never developed, has to win in South Carolina or his campaign — and, possibly, his chances to be Kerry’s vice president — will be finished.

Dean, it seems, won’t quit — ever. He thinks he can collect the money to carry on through the 10-state “national primary” on March 2 and possibly beyond.

Dean obviously is hoping that the “scrutiny machine” that has drilled so many holes in his candidacy will now cause Kerry’s ship to develop fatal leaks. You can expect Dean to help the process along.