Jump-starting his legislative career in the wake of a difficult White House defeat, Sen. John Kerry (D-Mass.) introduced his first major initiative Monday and is ready to begin a media push on children’s health issues later this week.

Pivoting off a presidential campaign in which he won a record number of votes for a Democrat yet disappointed many supporters with some crucial weaknesses as a candidate, Kerry is seeking to maintain a national leadership role by carving out a domestic policy agenda.

The first sitting Senator in 32 years to return to the chamber after losing a presidential general election, Kerry introduced a bill Monday to provide health insurance to every child, using the same legislation from his campaign as his first legislative priority.

Aides said Monday that the health care measure was the first in a series of issues on which Kerry would take “an active role,” including election reform and an effort to increase the size of the active-duty military.

“Anyone who knows John Kerry understands he’s a fighter, and now he’s fighting with all his energy for the issues that have been his passion for decades,” said Kerry spokesman David Wade. “He won’t allow anyone in Washington to retreat from the promise of health care or a foreign policy that makes America safe.”

Republicans regularly pilloried Kerry last year, saying that he had accomplished little legislatively in 20 years in the Senate — and that by glossing over his Senate tenure during his stump speeches, he was tacitly acknowledging the truth of that assertion.

Now, Kerry is making a concerted effort this week to garner media attention for his new push on health care. He’s set up a meeting in his office today with a group of top health care advocates and is using his appearance Thursday before Families USA as an official kick-off for his legislative campaign.

And on Sunday, Kerry is slated to appear on NBC’s “Meet the Press” — his first Sunday talk show interview since before Election Day.

The flurry of activity is certain to inspire speculation that Kerry is working to burnish his credentials for a possible second run at the White House in 2008.

No sitting Senator has ever won his party’s nomination, then lost the general election and gone on to win the nomination again.

But Wade said the focus is on the issues Kerry championed on the trail, not his next political decision.

“John Kerry has returned to the Senate to be a voice for the nearly 60 million Americans who voted for a new direction, and he’s joined by 2.7 million grassroots online advocates who believe in our agenda,” Wade said, referring to the massive e-mail address list his campaign compiled. That list is being melded into a leadership political action committee database. Supporters on the list will receive regular updates on Kerry’s agenda, both legislative and political, such as a recent e-mail blast touting his health care initiative.

The legislation also marks the first new step Kerry has taken to resume working with his senior Bay State colleague, Sen. Edward Kennedy (D), whose political influence was evident throughout the campaign, at times to the delight of Republicans eager to label Kerry a Northeastern liberal. Kennedy will be the lead player for Democrats on health insurance, but he has taken Kerry’s bill on children’s coverage and incorporated it into his broader bill.

Kerry’s bill was also introduced as a separate measure, and Kennedy signed on as the lead co-sponsor. In a sign of deference to the former Democratic nominee, the measure is being called the Kerry-Kennedy bill. Kennedy said he did not need to be the senior partner in their relationship.

“No, no, no, there’s room for everybody,” Kennedy said in an interview, adding that he hoped the children’s health issue was the first of several the two would be pushing in the years to come.

Sen. Rick Santorum (Pa.), the Republican Conference chairman, scoffed at the idea of Kerry re-starting his policy agenda this week.

“Re-starting his agenda? Don’t get me started. Some would question whether the ‘re’ belongs there,” Santorum said.

Aides to Kerry and Kennedy are realistic about their chances of getting any sort of Democratic legislation approved in a Senate with 55 Republicans. But they see the initiatives as an opening volley in a legislative battle that will likely run up to the 2006 midterms and possibly into the 2008 campaign season.

One senior Democratic strategist acknowledged that Kerry has been widely perceived as a Senator who tended to shy away from major domestic issues in the past, partly because Kennedy played such a major role on those issues. Kerry’s best opportunity to garner attention, the strategist said, came in issues not being pursued by Kennedy.

As a member of the Finance Committee, Kerry has a perch from which he can now dive into issues ranging from Social Security to health care and from taxes to trade. He was appointed to that critical panel after the 2000 elections but because he spent most of the past two years on the campaign trail, he has yet to emerge as a key player on Finance.

Because of electoral defeats and retirements, Kerry now ranks fifth in seniority among Democrats on the panel.

History has not been kind to sitting Senators who have run for president: Just two have won the White House. Not since George McGovern (D-S.D.) won the nomination in 1972 and then lost to President Richard Nixon has a failed presidential nominee returned to the Senate to immediately continue serving his term.

Some, however, returned to the Senate a few years after losing, including Barry Goldwater (R-Ariz.) and Hubert Humphrey (D-Minn.). And in recent decades, several vice presidential nominees have immediately retaken their Senate seats, including Joe Lieberman (D-Conn.) in 2001, Lloyd Bentsen (D-Texas) in 1989 and Bob Dole (R-Kan.) in 1977.