MANCHESTER, N.H., Feb. 18--In the dingy downtown hotel here, where John F. Kennedy began his drive to the White House eight years ago, George Romney's drive to the White House has been dealt another serious blow--and it might be fatal this time.

New Hampshire backers of Governor Nelson A. Rockefeller began a write-in campaign Saturday that could squash Romney in the key March 12 primary.

Chairman of the Rockefeller group, John A. Beckett, a professor at the University of New Hampshire, said in an interview that he has seen polls that show that a Rockefeller write-in effort could come close to or even surpass Romney, whose name is on the ballot.

Beckett did admit, however, that at this point--with only three weeks remaining--the campaign is "belated, illorganized, and under-financed."

Another problem is that the Rockefeller campaign is not being run by political professionals.

Only one week ago, on February 10, the slate of delegates met for the first time in a Concord motel and decided to start the campaign in earnest. On that same day, unofficial Kennedy and Reagan write-in groups decided to abandon their efforts.

The reason the Rockefeller delegates waited so long was a legal question. The New Hampshire Supreme Court cleared that up on February 9 when it ruled that a state committee for a write-in candidate could conduct a campaign and expend funds for the candidate without his consent.

The deadline for withdrawing delegate slates was 5 p.m. February 10--the day the delegates met in the motel.

Repeatedly urged to withdraw their names by letter, telegram, and personal envoy from Rockefeller, the delegates decided to stand fast and ignore their candidate's wishes.

They hope to pick up a large number of the 33,000 voters who went for Henry Cabot Lodge in the 1964 primary here, along with almost all of the 19,000 who went for Rockefeller in that race. This would give Rockefeller between 35 and 45 pe rcent of the vote.