Ronald Reagan says he believes he is the underdog against President Carter in the fall campaign, and he has done much since the Democratic National Convention to prove it.

Reflecting on the advantages of Carter's incumbency and a Democratic Party registration majority, Reagan said in an interview with The Washington Post: "I'm the one with a hill to climb."

The Republican presidential nominee suggested in the interview that the outcome of the race will depend on which candidate can make the other the issue. Reagan predicted that he will win if he can persuade voters that his proposals "are a viable alternative to the record of this administration, which is one of failure."

Reagan said he thought a head-to-head debate with Carter would help him make the president's record the issue. In the interview, Reagan also expressed no concern about his decline in the polls, and maintained that he was not letting stories about his supposed lack of intelligence get him down.

"I think any adult man knows his shortcomings and knows his strengths," Reagan said. "I've never claimed to be the originator of every idea we had in Sacramento, but I do think that I have the ability to recognize good ideas. I set the direction, the tone which we wanted to go [in California] . . ."

As to stories that he is too simplistic or not smart enough to be president, Reagan said: "It just goes with the territory. I don't let them spoil my life."

The interview was conducted before the worst of the political damage to the GOP campaign caused by some of Reagan's contradictory pronouncements on the Taiwan issue. But it came after Reagan had called the Vietnam war a "noble cause" in an address to the Veterans of Foreign War. The GOP candidate said he was satisfied with the response to that speech.

Reagan said he was enthusiastic about a friendly reception given by blue-collar shipyard workers in Philadelphia on Aug. 19. The blue-collar vote is a key target of the Reagan campaign.

"I feel good about the race, I feel good about what we're doing," Reagan said.

That feeling is not shared in all quarters of the campaign. One Reagan aide who was optimistic about the former California governor's chances three weeks ago said that the campaign had now lost its advantage. Another aide observed that the mistakes so far had been Reagan's own and that there wasn't much that aides could do about it.

Even those who do not feel that Reagan has yet been seriously damaged are worried about the lack of discipline he has displayed.

Reagan spent much of his so-called pre-campaign month of August preaching to the converted -- a county fair in Mississippi, two veterans' conventions, a meeting of fundamentalist ministers. In every instance Reagan lapsed into his banquet circuit habit of telling his audiences what they wanted to hear regardless of whether that helped his national campaign.

In Philadelphia, Miss., en route to an Urban League speech in New York, Reagan talked approvingly of "state's rights."

Before the VFW in Chicago, he blunted the impact of a serious speech on national security policy and arms control with his "noble cause" remark.

In Dallas, Reagan won audience approval, but not necessarily the confidence of voters he needs to reach, by suggesting that the theory of evolution is invalid.

While Reagan's staff often has been criticized for poorly scheduling or badly briefing the candidate, these mistakes -- if mistakes they were -- were Reagan's.

He made the final decision to go in Mississippi, against the advice of some aides who wanted him to travel straight to New York. He also wrote the offending passage into his speeches before the VFW convention and the ministers' meeting.

"The notion that Ronald Reagan is some kind of putty that can be easily shaped just isn't true," an aide said. "At least we've proved that." But there are those close to Reagan who are wondering whether the candidate can become disciplined enough to stand up against what is likely to be a well-disciplined reelected effort engineered by the Carter-Mondale committee. w

Already, there is a growing belief in the Carter camp that Reagan has made himself the issue.

The feeling in the Reagan campaign is that the anticipated debates between the two candidates and perhaps independent candidate John B. Anderson could well decide the election. Asked if he looked forward to a one-on-one debate with Carter, Reagan replied:

"I do look forward to it, not because of any contrast of ability in debating, but because I think the president cannot deny the record."

Reagan formally opens his campaign Monday at Liberty Park, N.J., with the Statue of Liberty on Liberty Island, N.Y., providing a photographic backdrop. He is to travel to Detroit later in the day, and is scheduled to return to Washington Tuesday night.

Reagan is to make a speech to B'nai B'rith in Washington Wednesday, and is not scheduled to return to the campaign trail until the following Sunday.

It is a light schedule for the 69-year-old candidate, who kept relatively busy in August -- much as Carter did four years ago when he was running against an incumbent president.

Reagan said in the interview that he has been bothered by a sore throat caused by an allergic reaction and aggravated by the dryness of airplanes and air-conditioned hotel rooms where the windows cannot be opened. He joked that what he needed was "an airplane with a window."

Despite his August difficulty and his belief that Carter has an incumbent's advantage, Reagan said he felt good about his decision to run for president and about his ability to do the job if elected.

"I have to feel good about it," Reagan said. "I think I can do the things I'm talking about doing. And I kind of feel that circumstances -- the things we did in California, in a situation similar to the one the country's in today -- gave me the experience to use it now."