Sen. John F. Kerry (D-Mass.) signaled a return to partisan warfare with President Bush yesterday in an e-mail to supporters in which he accused the administration of preparing a "right-wing assault on values and ideals" and called on Democrats to fight back against what he labeled Bush's extreme agenda.

Two weeks after delivering a generous concession speech that called for a lessening of the bitter partisanship that had marked the contest with Bush, Kerry picked up where the campaign left off and demonstrated his determination to be the leader of the opposition, in spite of his defeat.

"Despite the words of cooperation and moderate-sounding promises, this administration is planning a right-wing assault on values and ideals we hold most deeply," Kerry said in the message that was sent to about 3 million supporters who had signed up on the Kerry Web site during the campaign.

Kerry accused Bush of moving to eliminate debate and dissent from the State Department and CIA. He also charged that Bush's Cabinet was being remade "to rubber stamp policies that will undermine Social Security, balloon the deficit, avoid real reforms in health care and education, weaken homeland security and walk away from critical allies around the world."

The senator from Massachusetts promised to introduce legislation to provide health care to every child -- a scaled-back version of his campaign plan for expanded access to health care coverage -- when the Senate convenes next year. He asked supporters to sign a pledge to help him in his quest to pass the legislation in a body firmly in the Republicans' control.

The e-mail message represented a call to arms to Democrats. At a time when many Democrats are looking inward to examine the reasons for Bush's victory and to take stock of what they need to do to make their time in the political wilderness as short as possible, Kerry said Democrats should in no way give Bush a pass at the beginning of his second term. "This is not a time for Democrats to retreat and accommodate extremists on critical principles," he wrote. "It is a time to stand firm."

Kerry's decision to challenge Bush so directly suggested that he hopes to return to the Senate as a much more aggressive and forceful legislator than he was in the years before he ran for president. By staking out health care as the first issue on his post-election agenda, Kerry showed he is also willing to tread on turf long claimed by other Democrats, particularly Edward M. Kennedy, Massachusetts's senior senator, in whose shadow Kerry has spent his entire Senate career.

Kerry's message brought a quick end to the feel-good rhetoric that echoed from ceremonies marking the opening of former president Bill Clinton's library in Little Rock on Thursday. There, Bush and Clinton, along with former presidents Jimmy Carter and George H.W. Bush, shared praise of one another and offered up the symbolism of national unity and bipartisan cooperation.

Kerry's campaign-style rhetoric also was a jarring contrast to his concession speech on the afternoon after the election, when he told his despondent supporters that, in a congratulatory telephone call to the president, the two rivals had talked about "the desperate need for unity and finding the common ground, coming together."

Yesterday he told supporters that the Bush administration is hoping the opposition will disappear now that the election is over and said his message marks "the beginning of a second-term effort to hold the Bush administration accountable."

Stephanie Cutter, who was Kerry's campaign communications director, said that, Kerry's concession speech notwithstanding, it is up to Bush to demonstrate his willingness to govern in a way that begins to unify the country. "I think the nation has lived through four years of divisive policies and politics, and the president has a lot to prove to the American people that he wants to reach across party lines and do what's best for the country," she said. "John Kerry's making clear that, although he is ready to work together on strengthening America, he is not going to sacrifice what's important, like affordable and accessible health care."

Kerry noted that, while it is customary for a senator to seek a co-sponsor for legislation of the type he said he plans to introduce next year, he is reaching first to his base of Internet supporters for assistance. That was an indication that Kerry hopes to keep those supporters bound together behind him and his causes as he seeks to avoid the fate of other losing presidential candidates, whose power and support often dissipate quickly. Kerry's aides have said he is weighing whether to run again in 2008, and yesterday's message is consistent with that.

Sen. John F. Kerry, leaving the Capitol on Tuesday, issued a call to arms to Democrats.