At a joint appearance in Madison, Wis., Clark praised the Massachusetts senator for his "outstanding service" to the country in peacetime and in war.
"Request permission to come aboard, the Army's here," Clark, a retired four-star Army general told Kerry, a former Navy lieutenant, as the two stood together on stage.
Kerry said it's the first time he's had the privilege of saying "welcome aboard" to a four-star general.
"Both John and I served in Vietnam," Clark said, "and know what it is to be tested on the battlefield, fighting for your country. John Kerry never quit fighting for his country. …He will stand up to the Republican attack dogs and send them home licking their wounds."
In other endorsement news, Kerry appears set to win the backing of a key labor organization.
AFL-CIO spokeswoman Lane Wyndham tells CBS News that next Thursday in Washington, the group's president, John Sweeny, will make a strong recommendation to the AFL-CIO board to endorse Kerry for president.
If, as expected, the board accepts the recommendation, this will mobilize the full AFL-CIO, comprise of 64 unions and more than 13 million rank-and-file members, to start working on Kerry's behalf, Wyndham says.
Kerry looks to extend his lead in the Democratic presidential race when he clashes with Sen. John Edwards and Howard Dean in Tuesday's Wisconsin primary.
Polls in Wisconsin make Kerry a solid favorite. The latest American Research Group poll has Kerry at 53 percent among likely primary voters, compared to 16 percent for Edwards and 11 percent for Dean.
Kerry also now leads President Bush in a head-to-head match-up, 52 percent to 43 percent, according to a new Washington Post/ABC News poll. The poll finds a majority of Americans now believe Mr. Bush either lied or deliberately exaggerated pre-war evidence about Iraqi weapons of mass destruction.
Determined to slow Kerry's momentum, the president's re-election campaign has begun posting attack videos on its official Web site and e-mailing them to supporters around the country.
One posted on Thursday night mocks Kerry's frequent claim that he intends to show the special interests the door if elected.
"Sounds good," says the announcer, while the video shows a series of computer searches being launched. The results turn up newspaper stories that say Kerry raised "more special interest money than any other senator," and that "nominations and donations coincided."
"Kerry ... brought to you by the special interests," says the announcer. "Millions from executives from HMOs, telecoms, drug companies, Ka-ching. Unprincipled?"
The Kerry campaign labeled the Bush video a "smear tactic" and said it proves "the Bush White House is going to run a gutter campaign."
"The fact is, George W. Bush has taken more special interest money than any person in history," said Kerry campaign manager Mary Beth Cahill in an e-mail statement.
Kerry said he was ready for the GOP onslaught. "I've been pretty well vetted and examined from one side to the other," he told radio host Don Imus on Friday. "And I think that they're in for a surprise. I'm going to fight back. I am a fighter, and I'm ready to fight back."
There are 72 pledged delegates at stake in Wisconsin, and another 40 up for grabs in caucuses this Saturday: 24 in Nevada and 16 in Washington, D.C.
Kerry already has 557 delegates, according to a CBS News count, compared to 219 for Dean and 186 for Edwards. It takes 2,161 delegates to win the Democratic nomination for president.
Edwards, who has won one primary thus far, made clear that he intends to remain in the race regardless of his showing in Wisconsin.
After spending time in the state Thursday, he flew to Los Angeles to raise money. His schedule for Friday was a copy — in Wisconsin in the morning, then back to Los Angeles for fundraising.
Edwards has refrained from the type of attacks that other candidates have used in the race, and in an interview with The Associated Press, said the decision has helped him.
"The American people are tired of the same old politics that they've seen for so long, the attack politics," he said.
By contrast, Dean has become increasingly critical of Kerry in recent days as he looks for an upset in Wisconsin.
"We did not come all this way to change one Washington insider for another Washington insider," he declared to cheering students at the University of Wisconsin-Madison on Thursday.
Clark dropped out of the presidential race on Wednesday, offering words of praise for Kerry, Edwards and Dean.
He was the fifth contender to quit, after Sens. Bob Graham of Florida and Joe Lieberman of Connecticut, former Sen. Carol Moseley Braun and Rep. Dick Gephardt.
Moseley Braun endorsed Dean, Gephardt supports Kerry and Graham and Lieberman have yet to declare their backing for any of the remaining contenders.