In his first long campaign trip around the country, Ronald Regan answered a critical question about his candidacy while raising new doubts about his capacity to serve as president.

On a grueling, five-day, nine-state trip that ended in Los Angeles late Saturday, the 68-year-old Reagan younger challengers for the Republican presidential nomination.

The trip was designed to show that Reagan is more youthful than his years and it largely accomplished this purpose. He finished stronger that he began, with effective speeches to the Florida Republican presidential preference convention and to a friendly crowd in Cedar Rapids, Iowa, where the candidate is still remembered from 40 years ago as sportcaster "Dutch" Reagan.

But the critics who have contended, as long ago as Reagan's first presidential campaign in 1968, that he does not know enough to be president had new ammunition for their argument last week.

At a New York City news conference Reagan appeared unaware that the city is receiving loan guarantees from the federal government with considerable strings attached. His ignorance was all the more remarkable because aides said he had been briefed on the issue, which invariably arises whenever an out-of-town politican holds a news conference in New York. a

A day later, in Grand Rapids, Mich., Reagan, after long indecision, opposed pending legislation to provide similar loan guarantees to the trouble Chrysler Corp. He said there were other things that could be done to help the ailing automobile compamy, but didn't appear to have a clue as to what these other things were.

Returning home to California in his chartered campaign plane, Reagan expressed satisfaction with much of the first week's efforts but acknowledged that he hadn't answered either question very well.

"What I should have pointed out is that the system (of aid to New York) is in place, and it's working," Reagan said. "There's going to be no loss to the taxpayers on this one. I should have said that."

The Chrysler question, said Reagan, is a "hard and complicated one" that he now believes can be addressed by reorganizing the company. His issues advisers gave a similar analysis in considerably more detail in explaining to reporters what Reagan really meant to say in Grand Rapids.

Reagan's vunerability on issues that lie outside his basic speech may be all the greater because of the frontrunner status he enjoys within the Republican Party. Like Sen. Edward M. Kennedy (D-Mass.) among the Democrats, Reagan is surrounded by an entourage of press, staff and Secret service agents. Like Kennedy, he comes under the microscopic scrutiny of a presumed presidential nominee.

Last week Reagan conciously enhanced that scrutiny by giving on-plane interviews to almost everyone who asked. He made 10 speeches, held two news conferences, and met privately with key supporters at every stop.

Reagan's activity reflected both an attempt to deflect the "age issue" and a belief that he cannot sit still on his lead. If Reagan has any advantage over past frontrunners, it is in his recognition that leading in the polls can be exceptionally transtory.

"When you're out in front, the death watch begins earlier," Reagan said, "And it's true that you're kind of conspicuous out there. Almost anything that would be a momentary stubbed toe is going to be given much more importance. So I think you have the feeling of being very much on guard."

The guardedness showed early in the week in the tentative quality with which Reagan delivered some of his statistics-laden speeches. On several occasions he flubbed lines he normally delivers with a polished flair. But in California campaigns and again in 1976 Reagan had trouble hitting his oratorical stride in the early going, and his managers have history on their side when they predict he will improve.

History is also on the side of Reagan's basic campaign message, which speaks of the future in accents of the past.

"We have a rendezvous with destiny," Reagan said in his announcement of candidacy last Tuesday using words that stirred millions of Americans when they came from the lips of Franklin D. Roosevelt more than a generation ago.

That Roosevelt phrase has been a staple of Reagan's speeches for at least the last 15 years. And his language is studded with the phrases and metaphors of the generation, as when he talks of government taking from people "the fruits of their toil" or, from a Rudyard Kipling poem popular at that time, the words "East is East and West is West," mistakenly attributing the line to Mark Twain.

In Dorchester, Mass., Reagan declared proudly that he was "a lifelong member of the AFL," an organization that 24 years ago became the AFL-CIO. m

Except for his call for Puerto Rican statehood and his proposal for a North American accord among the United States, Canada and Mexico, Reagan's message in his first week of campaigning was a familiar one, and the canidate adknowledged witout apology that this was all right with him.

"Isn't it a little bit like a minister?" said Reagan. "You could say he's saying the same old line -- he's got a different sermon every Sunday but his theme is one and the same. My theme, as far back as I can remember, was a warning . . . The theme is that we continue to centralize authority, we continue with the government growing bigger -- and every time we do we're losing freedom."

If this is Reagan's oldest message, it is also the one he considers best suited to the mood of 1980. Republican audience on the campaign trail can expect to be hearing it many more times in the months to come.