Just down the hallway from the studio where the first Nixon-Kennedy debate was televised in 1960. Sen. Howard Baker was philosophizing about the travails of presidential politics.

"The real test," he said, "is whether you can stand the heat and not be turned off when you read those bad stories and whether you can keep going and convince your supporters that you still can make it."

Baker, who has weathered his share of unfavorable news stories in recent weeks, is convinced he still can win the Republican presidential nomination. But everywhere he goes these days he says, "We're playing catch-up ball."

His fortunes have undergone a fundamental change since he formally announced his candidacy Nov. 1 and claimed the contest for the GOP nomination was a three-man race among Ronald Reagan, John Connally and himself.

"It may still be a three-man race," the Tennessee Republican said in an interview here. "But the three men may have changed."

Baker, party leaders in a series of key states say, has become one of the "other" Republican candidates, a group that includes Sen. Bob Dole (R-Kan.), Rep. Phil Crane (R-Ill.) and Rep. John Anderson (R-Ill.).

"Howard announced, and sort of dropped off the cliff," says Dole.

The best evidence of this is a poll of Iowa Republicans reported Sunday by The Des Moines Register. It showed that Baker's support in the state had fallen from 23 to 11 percent since August, dropping him into fourth place among party members. Reagan led the poll with 50 percent: George Bush was second with 14 percent, up from 1 percent in August; John Connolly was third with 12 percent; Baker was fourth; and Dole was fifth with 3 percent. Crane and Anderson didn't register in the poll.

The "other" Republicans share a set of problems. They've all suffered staff upheavals and money shortfalls of one sort or another. They've all had trouble building grass-roots support. And they've all expressed frustration over inability to attract the kind of media attention they think they deserve.

But most of all, they are all waiting for Reagan, cautiously stalking his path, hoping the GOP front-runner will stumble, and their candidacies will suddenly be reborn.

"We're right here waiting with a big basket. We call it the open door policy. Our door is open," says Crane. Says Dole: "I tell people if they're tooking for a younger Ronald Reagan with experience, I'm available." Adds Baker: "Ronald Reagan is the name of the game as far as I'm concerned, and we've got to play catch-up ball."

The three, however, refuse to attack Reagan directly, other than suggesting that the 68-year-old former movie actor might be too old for the job. Crane and Dole simply don't want to offend Reagan supporters. Baker's refusal to attack is philosphical.

"I'm not going to climb over the dead bodies of my opponents to get the nomination," he says. "I'm not going to chew at their ankle or bite them in the back. Politics is mean and tough enough without that. Besides, I think that's the best politics."

Baker has a better chance than any of the "other" Republicans to challenge Reagan seriously. That he is among them is one of the surprises of the early rounds of the 1980 campaign.

He began the campaign better known and liked than any of his competitors except Reagan. His post as Senate minority leader gave him a natural national sounding board. But Baker's campaign has had trouble getting off the ground.

First, he tied his strategy to the debate over the strategic arms limitation treaty, thinking it would focus national attention on his role in the Senate. The debate, however, met delay after delay. And Baker, who originally had planned to announce his candidacy about July 4, finally announced Nov. 1.

His first week of campaigning ended in disaster when he was defeated by former ambassador George Bush in a straw ballot at a Republican convention in Portland, Maine. Baker was so confident that he'd win the nonbinding vote that he brought a placeload of reporters with him to witness the event. In the weeks since, his campaign manager, political director and key operatives in Iowa and Illinois have quit or been replaced.

Now, Baker says, "Perception is the name of the game . . . it's more important than what I do."

Crane and Dole have gone through similar traumas. Crane, a conservative from suburban Chicago, was so worried about how his campaign is perceived that he spent the first half of a speech at the National Press Club last month trying to dispel a series of "myths" about his candidacy. The biggest of the myths, he said, was that he was going to drop out of the race once he gets enough federal matching funds to pay off his debts. His proof: "It's a little hard to imagine why anyone in his right mind would maintain the kind of schedule I have if he was going to drop off."

"We've had feedback at the grass-roots level that a lot of rumors were circulating about me," Crane explained later. "I wanted to scotch them once and for all."

Every campaign, of course, has its problems, its internal battles and growing pains. The "other Republicans," however, have had more than their share of problems.

Dole's campaign, for example, has gone through four changes in its top management, and is in debt more than $210,000. Crane, who is more than $780,000 in debt, lost his entire senior staff in a bloody coup in early May.

"Staff problems slow you up. They're a setback," says Crane. "that's our big advantage. We had ours early."

Dole announced his candidacy May 14, and has hardly been heard of since. Initially, he hired a Washington consulting firm, Response Marketing, to run his campaign. He fired the firm mid-July, and has had someone new in charge about every six weeks since.

"It hurts," he says. "About the time you think you should have something going you look around and you find nothing is gone except the money and it's all gone.

"That's Bob's greatest failing -- his mouth," says one former Dole campaign manager. "In the time I spent with that campaign I could find no definite Dole constituency."

Parts of the problem with campaign staffs is that there are only so many experienced Republican political operatives in the country. The ones with national reputations are quickly hired. Others candidates have to turn to less experienced people. When something goes wrong, they often get blamed.

"Running for the Republican or Democratic nomination is roughtly equivalent to setting up a business where you employ 1,000 people, most of whom you've never met, where -- in a couple of months -- you set up branch offices in 50 states, and there is absolutely no job security after November," Baker told a fund-raising dinner here last week.

Baker, Dole, and Anderson also have spent much of their time this year on congressional duties. As ranking member of the Senate Finance Committee, for example, Dole has had to be in Washington during the entire debate on the oil profits tax bill.

He has been forced to sandwich campaigning into weekend and overnight trips. A couple of weeks ago he flew all the way to Iowa and back to attend a state Farm Bureau meeting in which he wasn't even allowed to give a speech -- just wave to the crowd. He came back exhausted, knowing the trip had not accomplished anything.

So what keeps a candidate at the bottom of the heap going?

"Whenever you get discouraged, you go out and do a couple of events and they charge you up," says Dole. "So you keep going."

"You just keep thinking that my break might come. That one event that might change the whole thing."