Kerry's strong victories in Virginia and Tennessee on Tuesday chased one Southern-bred rival from the race. Wesley Clark of Arkansas ended his bid after dismal third-place showings, while runner-up John Edwards of North Carolina said he would not quit.
As his remaining rivals scrambled for a last-ditch strategy to stop him in Wisconsin, Kerry retreated to his Washington home for two days of rest. He returns to the campaign trail tomorrow with stops in Wisconsin, which holds a primary next Tuesday, and Nevada, which holds caucuses on Saturday.
Kerry pledged to take it "one step at a time", first clinching the nomination and then targeting President George W Bush. But he spoke like a front-runner on Tuesday night, focusing more than half of his victory speech on the president.
"George Bush, who speaks of strength, has made America weaker weaker economically, weaker in our health care, weaker in education. And the truth is that he has made us weaker militarily by overextending the armed forces of the United States," Kerry said.
Howard Dean, once the race's high-flying front-runner, finished in single digits in Virginia and Tennessee, having skipped both states to campaign in Wisconsin.
Ohio's Dennis Kucinich and Al Sharpton, both winless, also have indicated that they intend to push on regardless of how well they do in upcoming primaries.
Edwards has just one win to Kerry's 12, but hopes Dean will drop from the race after Wisconsin and leave him as the only viable challenger to Kerry in the Super Tuesday contests on March 2.
But some Democratic leaders are calling for an end to the intra-party battle.
"At some point, perhaps sooner rather than later, I think Democrats need to unify behind John Kerry and refocus on winning in November," said Leon Panetta, who was Bill Clinton's chief of staff and is not affiliated with any candidate.
For Edwards and Dean, the temptation to stay in the race is strong because Kerry has not been tested by scandal or miscues thus far. Kerry's foes also point out that the crowded election schedule has not left much time for voters to take a second look at him.
Kerry has won 12 of 14 contests on the east and west coasts, in the Midwest, the Great Plains and the Southwest. Virginia and Tennessee had 151 pledged delegates at stake.
An analysis shows Kerry has piled up more than twice as many delegates as his closest pursuer.