Democratic front-runner John Kerry (search) vowed Saturday to aggressively counter Republican critics, drawing a stark contrast between his party and the GOP. "They're extreme. We're mainstream, and we're going to stand up and fight back," he said.

The Massachusetts senator, under fire from White House allies, sought to assure Democrats that he won't repeat mistakes of 1988 Democratic nominee Michael Dukakis (search), who responded cautiously to George H.W. Bush's assertions that he was a Massachusetts liberal.

"This week, George Bush and the Republican smear machine have trotted out the same old tired lines of attack that they've used before to divide this nation and to evade the real issues before us. Well, I have news for George Bush, Karl Rove (search), Ed Gillespie and the rest of their gang: I have fought for my country my whole life. I'm not going to back down now," Kerry, a decorated Vietnam veteran, said in remarks prepared for delivery Saturday night to Virginia Democrats in Richmond.

Rove is President Bush's top political adviser. Gillespie, head of the Republican Party, has borrowed from the 1988 play book to label Kerry a Massachusetts liberal with a "long record in the Senate is one of advocating policies that would weaken our national security."

As votes were being counted in Michigan and Washington state caucuses, Kerry looked beyond his bickering Democratic rivals to the fall election. Kerry is trying to convince voters that he's above party wrangling and ready to focus on Bush.

"This is one Democrat who's going to fight back, and I've only just begun to fight," he said. "George Bush, who speaks of strength, has made America weaker — weaker economically, weaker in health care and education. And the truth is George Bush has made us weaker militarily by overextending our forces, overstraining our reserves, and driving away our allies."

Earlier, Kerry said he'll campaign against Bush in the South, dismissing Republican assertions that he is too liberal and out of touch to win in Dixie.

"This administration is busy trying to paint everybody else as out of touch, out of synch, somehow out of the mainstream," Kerry said at a Nashville university. "But let me tell you something: I'm not worried about coming down South and talking to people about jobs, schools, health care and the environment. I think it's (the president) who ought to worry about coming down here."

Kerry made the remarks at the beginning of a weekend swing through Tennessee and Virginia, the two states holding elections Tuesday. His main rivals, John Edwards and Wesley Clark, need a victory in the Southern primaries to keep their races afloat.

Kerry has gotten himself in trouble by suggesting that a Democrat can win the presidency without carrying a Southern state. While that may be mathematically possible, even Kerry's own advisers say it was indiscreet to talk of putting an entire region off the Democrats' political map.

In his Virginia remarks, Kerry said Democrats represent the mainstream, Republicans the "extreme," on a number of issues, including tax cuts, fiscal responsibility, health care, violence against women, the federal judiciary, civil liberties and national security.

"Our opponents now say they want to campaign on national security. But this is the same administration that slashes health care for veterans, tries to cut combat pay for our troops in the field, makes injured soldiers pay for their own hospital meals, and leaves soldiers on their own to buy high-tech flak jackets," he said. "We are fighting for the mainstream value of a stronger America and for the ideal that the first duty of patriotism is to honor those who wear and have worn the uniform of the United States."

Kerry, a Navy veteran, said: "We all saw George Bush play dress-up on an aircraft carrier. Well, I know something about aircraft carriers for real. And if George W. Bush wants to make national security the central issue in this campaign, I have three words for him I know he understands: Bring it on."

It's a standard line in his stump speeches, one repeated in unison by cheering crowds.

Kerry's rivals have all but conceded three elections in Michigan, Washington state and Maine, and Kerry hopes to knock Edwards and Clark from the race with a sweep Tuesday.

Beyond that, the Democratic field, which still includes Dean, is moving to a potentially decisive showdown Feb. 17 in Wisconsin.

"When you add up the real deficits in our nation ... it's not just measured in money, it's measured in the hopes that are dashed," Kerry said in a speech to several hundred supporters Belmont University in Nashville.

Kerry said that when he engages with Bush, "it will be clear across this land that the one person who deserves to be laid off is George W. Bush."