WASHINGTON — Already unprecedented in size, the presidential field of candidates will grow by one more today, as the Republican governor of Arkansas plans to add his name to a list of likely hopefuls that now numbers more than 20 from both parties.

Governor Huckabee said yesterday that he would file papers today to form an exploratory committee, following closely on the heels of another GOP contender seeking the party’s conservative mantle, the former chairman of the House Armed Services Committee, Rep. Duncan Hunter of California, who announced his bid in South Carolina last week.

Messrs. Huckabee and Hunter are looking for a niche in a GOP slate in which the early front-runners, Senator McCain, Governor Romney, and Mayor Giuliani, all have shaky credentials among some social conservatives.

The Democrats have an equally long list of contenders. “I’m the 800th candidate,” Senator Biden of Delaware quipped yesterday on the ABC News show “This Week.” Mr. Biden has repeatedly said he intends to run in 2008, but he will “officially” declare his candidacy on Wednesday.

More announcements could be on the horizon. Governor Pataki has long been considering a run, and he gave a speech here on Friday about Iraq that sought to thrust himself back into the foreign policy debate. And Senator Hagel of Nebraska, an anti-war Republican, says he is also thinking about 2008, although his effort could come as an independent. He even joked in an interview with the Washington Post that he could team up with Mayor Bloomberg in a maverick try for the White House. Mr. Bloomberg has insisted he is not running, but he and his aides have fed speculation by refusing to rule it out.

The ballooning number of applicants for the presidency has scholars reaching back to 1952 or even 1928 to find a suitable comparison. In 1952, President Truman did not seek reelection, leaving an open Democratic slot (eventually won by Adlai Stevenson), while General Dwight Eisenhower defeated Senator Robert Taft for the Republican nomination. In 1928, President Coolidge did not seek reelection in a campaign won by Herbert Hoover.

While a few more recent races have drawn a large pool of candidates for one party – 1988 was a big year for Democratic hopefuls, for example — none has been a race in which neither the incumbent president or vice president is running.

That leaves an open field, and one that is not likely to occur again any time in the near future. “You have a large number of people who have dreamed of being president and realize that this may be their last opportunity,” a political science professor at the University of Virginia, Larry Sabato, said.

On the Democratic side, Senator Dodd of Connecticut and Senator Biden of Delaware are each veteran lawmakers who have eyed the presidency before.

And 2008 will likely be the final chance for Senator McCain of Arizona, a leading Republican who is 70 and already sought the White House in 2000 before losing to George W. Bush.

But with more than 20 candidates seeking the same limited pool of money and attention, how long will they all be able to stay in the race? “This field will shrink to a more realistic number, but you’re still going to have a huge number of candidates from both parties,” a political consultant and the associate dean of Boston University’s College of Communications, Tobe Berkovitz, said.

While a few hopefuls who have formed exploratory committees may ultimately decide not to run, most will stay in at least through the first ballots next year in Iowa, New Hampshire, South Carolina, and Nevada, after which the field will likely dwindle considerably, analysts said.

From the front-runners to the longest of the long shots, every candidate has a scenario in mind, analysts said, in which the pieces fall into place for them to make a strong showing in one or more of the early primaries, gain momentum and a surge of donations and national press attention that allows them to compete and even win the larger states that follow.

“All of the scenarios are remotely possible, which is why you have so many candidates getting in and staying in,” a professor of political science at Hunter College, Andrew Polsky, said. Or, as Mr. Sabato put it: “It’s January of 2007, and anything’s possible.”

The increasingly likely possibility that the two parties will restructure the voting calendar to bunch more primaries earlier and closer together could further dampen the hopes of second-tier candidates, Mr. Polsky said. Those candidates would normally look to spend money and organize on a state-by-state basis, allowing them to build momentum — and funds — incrementally. “You’d have to set them all up at the same time” under the new scenario,” he said. “A very expensive proposition.”