Every day, it seems, a new candidate or two jumps into the 2008 presidential race. No fewer than 26 contenders may end up competing. Fourteen Republicans and 12 Democrats are either running, exploring, or thinking about it.

Why is the field so crowded? Because 2008 is the most wide-open presidential contest in more than 50 years. It's the first race since 1952 in which neither an incumbent president nor vice president will be vying for the nomination. All of this year's wannabes are saying, "I'd better go for the big prize now because the situation is never going to look this good again."

That's true even for Republicans, who have to bear the burden of their party's unpopular president. George W. Bush looks more and more like a lame duck. His job-approval rating is lower than that of any other president since 1952, as measured at the beginning of their penultimate year in office. Vice President Cheney's decision not to run means that no Republican candidate is carrying the Bush banner.

Three top-tier candidates have emerged in each party. When the January ABC News/Washington Post poll asked GOP voters their preference for the nomination, two contenders earned double digits: former New York City Mayor Rudy Giuliani (34 percent) and Sen. John McCain of Arizona (27 percent). Former Massachusetts Gov. Mitt Romney, at 9 percent, is tied for third place with former House Speaker Newt Gingrich of Georgia.

Giuliani, McCain, and Romney share some liabilities: Conservatives find their positions on social issues either unacceptable or unreliable. And all three have tied themselves to Bush's Iraq policy. Specifically, they have all endorsed the troop buildup. Unless the military situation improves dramatically, the Iraq war could drag these candidates down. That is one major reason that at least 11 other Republicans—particularly Sen. Sam Brownback of Kansas, a social conservative who opposes Bush's Iraq buildup—think they may have a chance.

The top tier of Democratic candidates includes Sen. Hillary Rodham Clinton of New York (who clearly dominates the race, with 41 percent support among Democratic voters in the ABC/Post poll), followed by Sen. Barack Obama of Illinois (17 percent), and former Sen. John Edwards of North Carolina (11 percent). The question hanging over the Democratic race is, Can Hillary get elected? Indeed, on the day she declared, "I'm in," on her Web site, Mark Penn, her chief strategist, posted a memo addressing that question: "People are always asking, 'Can Hillary Clinton win the presidency?' Of course she can. In many of the polls out today, she is already winning."

That's true. But when Clinton is pitted against Giuliani or McCain, the race looks close. She leads McCain by 5 points and Giuliani by 2 points in the ABC/Post poll.

Democrats who worry about Clinton's electability may find themselves asking the same question about Obama and Edwards. All three must contend with the commander-in-chief problem: Do they possess the experience to be a credible military commander? None has served in the military, although Clinton has made a serious effort to acquire military expertise as a member of the Senate Armed Services Committee and a Pentagon task force.

Overall, Clinton starts out with significant advantages. She can raise big money. She can compete with Obama for the African-American vote; the ABC/Post poll shows her winning a majority of the vote among nonwhite Democrats. She has the best political adviser money can't buy: her husband. And she has been tested. "I know how Washington Republicans think, how they operate, and how to beat them," Clinton said in her announcement video.

What will be decisive in the 2008 contest? Most likely, the polls. They will answer the question, "Can Hillary win?"

If the answer is yes, Democrats are likely to nominate her. The polls will also answer the question that hangs over the GOP race: "Can anyone beat Hillary?" As long as Giuliani and McCain look like the Republicans best positioned to stop a Clinton restoration, conservatives may swallow their reservations about them. The perceived ability to keep Hillary Clinton out of the White House is a powerful conservative credential.