A conservative group that has mounted an expensive advertising campaign to unseat Sen. Paul Sarbanes (D-Md.) yesterday released a poll showing that Sarbanes would be vulnerable to a challenge from Rep. Marjorie Holt (R-Md.).

The poll showed that Sarbanes, a liberal Democrat in his first term, led Holt in a trial heat by 43.7 to 37.5 percent with 18.8 percent of those surveyed undecided.

Sarbanes, according to the poll commissioned by the National Conservative Political Action Committee, is largely an unknown quantity to Maryland voters surveyed and is given a good or excellent job rating by 37.4 percent of them.

Sarbanes registered an extremely low "negative" in the poll, however, a category considered critical by professional politicians. Asked what they liked least about the senator, 75.4 percent of 603 voters surveyed couldn't come up with anything specific. Asked what "you like most about Sarbanes," 53.7 percent of those survey said, "don't know."

The telephone poll was done for NCPAC by Arthurf Finkelstein, a pollster who specializes in conservative candidates, in March before NCPAC launched a television ad campaign portraying Sarbanes as "the biggest spender in the Senate."

Sarbanes' press secretary, Bruce Frame, yesterday said the senator has not conducted a poll of his own, and would not comment on the NCPAC survey withoutg studying the questions. "You can get a poll to get any kind of answers you want," he said.

Sarbanes, the press spokesman added, believes the $100,000 ad campaign has not damaged him. "The senator doesn't think people in Maryland are going to be manipulated by a slick ad campaign put together by people outside the state."

NCPAC's poll, however, coincides with soundings taken by a liberal group, the National Committee for an Effective Cogress, which supports Sarbanes. "Obviously, Sarbanes is vulnerable," Russell Hemenway, the committee's executive director, told a group of reporters Wednesday night.

NCPAC has announced plans to spend $450,000 in the 1982 Maryland Senate race. The reelection chances of three other leading Democrats NCPAC has targeted in 1982, House Majority Leader Jim Wright (D-Tex.), House Budget Committee chairman James R. Jones (D-Okla.) and Ways and Means Committee Chairman Dan Rostenkowski (D-Ill.), were said to be good in polls by Finkelstein.