By John Whitesides, Political Correspondent

NEW YORK (Reuters) - Democratic presidential nominee Barack Obama warned his supporters to guard against overconfidence on Thursday as he and underdog Republican rival John McCain opened a 19-day sprint to Election Day.

The two candidates hit the campaign trail -- Obama raising money in New York and McCain holding a rally in Pennsylvania -- after their third and last presidential debate on Wednesday, a testy face-off that made an Ohio plumber famous.

So far, all the stars seemed to be lining up in Obama's favor. He leads in national opinion polls and in many of the battleground states where the November 4 race will be won or lost.

Traders betting on future events in the political prediction markets are overwhelmingly predicting an Obama victory, giving the Illinois Democrat a better than 80 percent chance of winning.

But Obama pointed out to deep-pocket contributors at a fund-raising breakfast in Manhattan that he was supposed to win New Hampshire last January in the Democratic primary but lost the state to Sen. Hillary Clinton.

"For those of you who are feeling giddy or cocky or think this is all set, I just have two words for you: New Hampshire," Obama said.

"I've been in these positions before when we were favored and the press starts getting carried away and we end up getting spanked," he said.

NEW POLITICAL STAR

The new star in U.S. politics -- at least for a news cycle or two -- is Joe Wurzelbacher, "Joe the plumber," who told Obama at a campaign stop that he wanted to buy a small plumbing business in Holland, Ohio.

Joe came up about two dozen times during the debate as each candidate argued that their prescriptions for America's economic ills would help the plumber best.

Wurzelbacher was all over morning television shows and was not saying who he would vote for, but he sounded like a McCain backer.

"McCain came across with some solid points, and I was real happy about that," he told the Toledo Blade newspaper.

Although McCain went after Obama aggressively in the final debate, it might not have done much to change the shape of the race in its final stages.

Karl Rove, the architect of President George W. Bush's two electoral victories and now a political pundit, wrote in The Wall Street Journal on Thursday that McCain has difficult but not impossible odds.

"If Mr. McCain succeeds, he will have engineered the most impressive and improbable political comeback since Harry Truman in 1948. But having to reach back more than a half-century for inspiration is not the place campaign managers want to be now," Rove wrote.

The two candidates will be on the same stage again on Thursday night when they give speeches at the Al Smith charity dinner, a political tradition in Manhattan named for the former New York governor and a regular stop for presidential candidates.

(Additional reporting by Andy Sullivan; writing by Steve Holland; editing by David Wiessler)