MANCHESTER, N.H., FEB. 11 -- Republican presidential candidate Alexander M. Haig Jr. plans to abandon his campaign and endorse Senate Minority Leader Robert J. Dole (R-Kan.) at a news conference here Friday, a well-informed Republican source said tonight.

Haig has scheduled a news conference for 10 a.m., but his New Hampshire campaign staff said it did not know the purpose. Neither Haig nor his senior aides could be reached for comment.

Polls show Haig in last place in the six-man GOP presidential field. And he has all but publicly said that he has no chance to be nominated. His decision to withdraw from the race and endorse Dole four days before Tuesday's crucial New Hampshire primary would be a final, parting shot at Vice President Bush, whose front-runner status has been crumbling since Dole won the Iowa caucuses Monday.

Haig, a former secretary of state in the Reagan administration, has made no secret of his dislike for Bush. At the GOP presidential debates, he has been Bush's leading critic. In a speech in New York late last year, Haig said that Bush had failed every major assignment given him by President Reagan.

Bush has boasted that he has been Reagan's "copilot." But Haig charged that, in the missions Reagan gave Bush, the copilot either "wasn't in the cabin," flew in the "wrong direction" or "crashed."

During the first Republican presidential debate last year, Haig stunned Bush by demanding an explanation of his role in the Iran-contra affair.

The timing of Haig's announcement seemed aimed at giving Dole's momentum from Iowa an extra boost going into the primary. While Haig did not enjoy a large following, his endorsement of Dole would "buttress Dole's case that he is the viable alternative to Bush," a GOP source said.

"It reinforces the conservative credentials of Bob Dole," he added.

Details of the negotiations leading up to Haig's decision are not known. Sources said discussions were held here Wednesday, including talks between Douglas Scamman Jr., speaker of the New Hampshire House and a Dole supporter, and David Young, a member of Haig's New Hampshire staff.

Dole has been tentatively scheduled to leave New Hampshire Friday for a southern Republican meeting in New Orleans. However, he is not scheduled to leave the state until late morning, which would allow him to appear at the Haig news conference.

Staff writer James R. Dickenson contributed to this report.