Rep. Marjorie S. Holt, the conservative Maryland Republican who has spent a year preparing to challenge Democratic U.S. Sen. Paul Sarbanes, backed out of the 1982 race yesterday, saying she prefers her "comfortable" seat in the House of Representatives.

"I think this is the happiest day of my political life," Holt, 61, said of her decision to forgo a Senate race and run for a sixth term as Maryland's 4th District congresswoman.

"I think I could beat Sarbanes but it would be a very tough battle -- and it would be very, very close," she said. "I realize what a wonderful seat I have in the House."

Although Sarbanes is at the top of the GOP's 1982 hit list, any Republican challenger is expected to have a difficult time because Maryland is a heavily Democratic state -- one of the few to vote for Jimmy Carter in 1980.

Holt has represented Anne Arundel and southern Prince George's counties in Congress since 1972. In order to run against Sarbanes, she would have to give up substantial seniority and a possible future subcommittee chairmanship should Republicans gain control of the House. "This," she said,"cannot be sacrificed lightly."

Holt's announcement, which she made in a conference room of a state office building, came after most Republican leaders in Maryland urged her to hold onto her seat. Her decision leaves Prince George's County Executive Lawrence Hogan as the only major GOP figure now in the race. Hogan spread the word a few weeks ago that he will not seek reelection to his county post but will run instead for the Senate.

Yesterday the Republican county executive, who served in Congress from 1968 to 1974 and then lost a GOP primary for governor, said Holt made a "wise decision" to stay in the House.

Hogan said "it obviously helps" to have Holt out of the race since the two of them draw from the same conservative base. "A lot of people conservative contributors were put in a very agonizing position," said Hogan, whose own positions in Congress were as conservative as Holt's.

While Holt's decision momentarily leaves a clear field for Hogan, it is quite likely that other candidates will emerge to challenge him. Hogan has not been very popular with the hierarchy of the Republican Party and some state GOP leaders, aware that Holt might not run, have been looking for other candidates.

One possible candidate is former U.S. senator J. Glenn Beall Jr., who served one term in the Senate before losing to Sarbanes in 1976 and who ran unsuccessfully against Gov. Harry Hughes in 1978. Beall, now back at his Western Maryland insurance business, said yesterday that he is thinking of running but would like to see a poll done before committing himself.

"If you ask me whether I'd like to be a United States senator, I'd say sure, but I'd like to find out what a poll says," Beall said.

Others who have been mentioned as possibilities are Dallas Merrell, who ran against U.S. Sen. Charles McC. Mathias in the 1980 GOP primary; Glenn Beall's brother George Beall, the former U.S. attorney who prosecuted Spiro T. Agnew; former Iranian hostage Bruce Laingen and conservative columnist George Will. None of them could be reached for comment yesterday.

Whoever runs, however, will have to deal with the same problems that eventually persuaded Holt not to run: an incumbent who is already campaigning throughout the state; a 3-to-1 Democratic registration, and what Republicans call "the Baltimore problem." Baltimore, whose votes usually determine the course of a Maryland election, is dominated by traditional Democratic constituencies: labor unions, ethnic populations and blacks.

Holt spent a lot of time trying to make inroads into Baltimore, meeting with Mayor William Donald Schaefer and bringing federal officials to see the city. But as supporter Edward Mason, Republican leader in the state Senate, said yesterday, "She didn't pick up the momentum that she thought she should've picked up" in Baltimore, Sarbanes' hometown.

Holt said yesterday that her polls indicated she could overcome that problem if she campaigned nonstop for the full year between now and the 1982 election. "If I did everything right and I didn't make any mistakes, then I could beat him," Holt said. However, the prospect of such a year was unappealing and already, before the campaign had officially begun, a few mistakes had occurred.

One in particular was a widely publicized comment that Holt made last week when several members of the NAACP picketed her annual bull roast with signs that read, "While Republicans feast, the poor don't eat." Holt said at the time that "some of those ladies look like they could lose some weight," a comment which she privately told friends was a bad mistake.