Two members of Congress have asked Attorney General William French Smith to investigate actions of the National Conservative Political Action Committee that they consider illegal. One member of the House has accused NCPAC's chairman, John T. (Terry) Dolan, of attempted bribery and blackmail.

The Justice Department has not decided whether to initiate formal investigations, a spokesman said yesterday.

The charge of attempted bribery and blackmail was made by Rep. Stephen L. Neal (D-N.C.) and was based on a letter he received from Dolan. In the letter, dated last Friday, Dolan wrote:

"If you will make a public statement in support of the president's tax-cut package and stage that you intend to vote for it, we will withdraw all radio and newspaper ads planned in your district. In addition, we will be glad to run radio and newspaper ads applauding you for your vote to lower taxes."

The federal statute regarding bribery of public officials makes it illegal to give or offer "anything of value to any public official . . . for or because of any official act performed or to be performed. . . ."

A senior federal prosecutor said yesterday that the wording in the letter was "very, very close to the line" dividing acceptable behavior from an illegal attempted bribe. The prosecutor, who asked that he not be identified and said he had no direct familiarity with the matter, added that the Justice Department would have to conclude that there was a criminal intent involved to press any charges.

The second complaint against Dolan and NCPAC came from Sen. Daniel Patrick Moynihan (D-N.Y.), who received a letter from the NCPAC chairman last month urging him to vote against a proposal to close certain tax loopholes involving "straddles." Moynihan forwarded the letter to William F. Hildenbrand, secretary of the Senate, asking if Dolan was registered as a lobbyist.

He was not, and Hildenbrand forwarded the matter to Justice Department for possible investigation.

Dolan and NCPAC are familiar with political controversy. The group has specialized in running negative radio and television ads attacking liberal Democrats. NCPAC and Dolan have been criticized by some Republicans, but in May President Reagan's political adviser, Lyn Nofziger, endorsed the group's activities.

NCPAC's commercials create "a climate of doubt" about the Democrats and are, "on balance," a help to the administration, Nofziger said.

Dolan was traveling outside Washington yesterday and could not be reached for comment.