WASHINGTON – Responding to worried Democratic colleagues urging him to step up the attack on President Bush, Senator Kerry yesterday broke with the tradition of presidential candidates remaining silent during an opponent’s convention and returned to the campaign trail with a speech claiming that “extremism has gained momentum” as a result of Mr. Bush’s mistakes in Iraq.

“When it comes to Iraq, it’s not that I would have done one thing differently, I would have done almost everything differently,” said Mr. Kerry in an appearance before the national convention of the American Legion, a gathering Mr. Bush spoke to on Tuesday.

Mr. Kerry saluted the audience of veterans as he came to the podium and again saluted as he concluded a speech in which he challenged the president’s anti-terror policies and accused the administration of failing to listen to the advice of the military on the number of troops needed in post-Saddam Iraq.

He took issue with Mr. Bush for venturing into Iraq without United Nations backing and said “I would never have diverted resources so quickly from Afghanistan…I would have built a strong, broad coalition of our allies around the world…I would never have gone to war without a plan to win the peace.” As a result of administration policy “today’s terrorists have secured havens in Iraq that were not there before,” he said.

Shortly after Mr. Kerry’s speech, White House strategist Karl Rove accused the Democrat of having tarnished the endeavors of Vietnam vets with his anti-war protests and testimony three decades ago. Mr. Rove said: “I do know that John Kerry has said, ‘judge me by the record’ and spent a lot of time talking about his service in Vietnam, which we ought to honor. There are not going to be ads and such by the Bush campaign about this, but it’s something that the American people have a right to take into consideration.”

Mr. Rove’s comments prompted a blistering response from senior Democrats, including a former senator from Nebraska, Bob Kerrey, who said that the remarks show there is coordination between the Bush campaign and the anti-Kerry Swift Boat Veterans for Truth, who have sought to cast doubt on the Democrat’s war record.

Mr. Kerry’s appearance came amid Democratic jitters about the fortunes of the Massachusetts senator’s campaign. His eroding approval ratings in a series of recent public and private party polls has prompted internal party criticism of the Kerry effort, leaving leading Democrats and party strategists anxious that the president may be gaining a momentum that will be difficult for Mr. Kerry to reverse.

Party critics have urged Mr. Kerry to respond more quickly and forcefully to Republican attacks on his war record and they dub as a mistake his decision to stay aloof initially from the fray triggered by accusations from the Swift Boat Veterans for Truth. With polls now showing that voters believe the president is better qualified than Mr. Kerry to be the commander in chief, some party strategists argue that the Democratic candidate should have struck back earlier.

The Swift boat accusations “scored a lot of political points,” said a Democratic fund-raiser, Frank Schreck. He has publicly called for Mr. Kerry to compare the candidates’ war records and to “stand up there and say, ‘He chose to have his father get him out of harm’s way while I volunteered to risk my life.'”

Other Democrats anxious about the way the campaign is shaping up disagree and say Mr. Kerry has to criticize Mr. Bush on the economy and bread-and-butter issues that traditionally favor the Democrats. “My sense is the Swift boat stuff has been a major distraction, to say the least, for the campaign, and they need to get back to hammering him every day on the economy and health care and the management of the war,” said Democratic consultant Bill Carrick.

Senior Democratic sources tell The New York Sun that Mr. Kerry has become increasingly frustrated with his inner circle of election advisers, but that although he is keen to shake things up, he has decided not to replace either his campaign manager, Mary Beth Cahill, or the communications director, Stephanie Cutter, preferring instead to add people to the key group of advisers.

Yesterday, the Kerry camp announced that former Bill Clinton advisers Joe Lockhart and Joel Johnson have joined the campaign for the final two-month stretch. Mr. Lockhart will travel with Mr. Kerry.

The Kerry camp is also preparing an advertising blitz for the immediate wake of the GOP convention. That is another unusual electioneering move for a presidential campaign – normally an advertising onslaught is started a little later in the September of an election season. The campaign says it will buy $45 million worth of commercial time in 20 states for now through Election Day. The first ads will start going out this weekend. Ten states have been earmarked for the early push and another 10 will be added as the weeks go by.

The Democratic Party yesterday aired a new 30-second ad in battleground states, accusing Mr. Bush of “a failure in leadership.”

Meanwhile, as Republicans and Democrats traded charges, the Bush-Cheney campaign filed a lawsuit in Washington, D.C., to try to force the Federal Election Commission to halt independent anti-Bush groups from spending millions of dollars in the presidential race. The campaign claims the spending is illegal and violates campaign finance laws.