Obama vs. Clinton vs. Kerry in '08?

Sen. Hillary Rodham Clinton, D-N.Y., was taking part in a televised debate Sunday that was supposed to be about the New York Senate election two weeks away. But the first question was about the presidential election two years away.

WABC political reporter Dave Evans asked Clinton if she would repeat the pledge she made when she first ran in 2000 to serve her full, six-year Senate term. Doing that would rule out a run for the White House in 2008 and Clinton was not willing to repeat her pledge.

"I can't make a decision now" about running for president, she said. "I have made no decision. But if that concerns any voter, they should factor that into the vote they make."

Don't look now, but even though the 2006 congressional midterm elections haven't even been held, the 2008 presidential campaign is already underway.

"Presidential campaigns basically never stop," said Mark Halperin, ABC News' political director and co-author of "The Way to Win: Taking the White House in 2008."

This campaign is shaping up as the first since 1928 in which there will be no sitting president or vice president running.

"That produces a lot of aggressive, early activity," Halperin said.

Clinton's refusal to rule out a 2008 run is not surprising; she's seen as the Democratic frontrunner.

More surprising was the declaration from freshman Sen. Barack Obama, D-Ill., who has been campaigning for Democratic congressional candidates and had earlier said he was not inclined to run.

"Given the response that I've been getting over the last several months, I have thought about the possibility" of running for president, Obama said on NBC's "Meet the Press." "I have thought about it over the last few months."

Obama is riding the crest of a publicity wave right now, with a new book out and his face gracing the cover of magazines ranging from Time to Men's Vogue. His first turn in the national spotlight came in 2004 when Democratic presidential nominee John Kerry asked him to address the Democratic National Convention.

Now, Kerry is thinking about another run, and told ABC's George Stephanopoulos on "This Week" that he'd welcome the competition from Obama.

"If he thinks he's ready to run for president and wants to run, and I've made a decision, we'll go out there and have a great contest," said Kerry, who said he'll decide whether to run sometime after the midterm elections.

A race between Clinton and Obama would pit two very different figures against each other.

Clinton, a former first lady, has been on the national stage for nearly 15 years and has some prominent opponents.

Obama is a newcomer whose publicity has been virtually all positive -- although even he acknowledges that will change if he becomes a candidate.

Obama has already tested the waters in Iowa, the state with one of the earliest primary contests. And while Hillary Clinton hasn't made the trip, another politician named Clinton -- her husband, former President Bill Clinton -- has.

"Hillary Clinton is a huge front-runner in this race, and if Barack Obama follows through and gets in, he becomes a powerful force," said Halperin. "A lot of other candidates are going to be pushed to the side in media attention, money and political support if you have these two titans running for president."

It's not just Democrats vying for a head start. Republicans with visions of the Oval Office dancing in their heads have also beaten a path to Iowa. The most frequent visitors are little-known names trying to be better-known names.

A trio of governors has set foot in the state again and again -- Mitt Romney of Massachusetts, who's made nine trips, George Pataki of New York and Mike Huckabee of Arkansas, who have each made seven forays.

The visits appear to be paying off for Romney, who is burnishing his conservative credentials and is widely seen as second to the more moderate John McCain.

"John McCain is a formidable front-runner, but there are a lot of doubts about him within the Republican Party," said Halperin. "Mitt Romney has been the most aggressive and the most successful in playing off some of those doubts and positioning himself as McCain's chief challenger."