Maryland Gov. Harry Hughes will abandon his netural position in the Democratic presidential campaign later this week by joining a cast of more than 100 state politicians endorsing the reelection of President Carter, party officials said yesterday.

Hughes, an early Carter supporter in 1976, has agreed to serve as honorary chairman of the president's primary campaign in the state. He will be flanked at an Annapolis news conference for Carter on Thursday by Baltimore Mayor William Donald Schaefer, Montgomery County Executive Charles Gilchrist and scores of state legislators and county officials.

The vast support for Carter among the state's Democratic establishment this year is in marked contrast to four years ago, when then-governor Marvin Mandel and most party officials supported California Gov. Edmund G. (Jerry) Brown in the state's primary.

Brown, who resoundingly defeated Carter in Maryland that year, is challenging him again, but has yet to garner the endorsement of any elected official in the state.

"I guess a lot of people have seen the light," said Arnie Miller, who helped direct Carter's Maryland campaign in 1976 and subsequently landed a job in the White House. "This administration has been a great service to the people of Baltimore . Things are looking good in Maryland."

Although there are few surprises on the list of Carter backers, one party official said that "the depth and breadth of support, in terms of elected leaders, is broader than most people expected."

Maryland supporters of Sen. Edward Kennedy, who had been organizing since early last fall for the state's May 1o primary, attempted to be optimistic in spite of the blow.

"Endorsements, as President Carter said four years ago, are not the name of the game," said James Flug, Kennedy's coordinator for Maryland. "The thing that counts is the candidate and a functioning organization."

Flug said the Carter list consisted mainly of officials dependent on the federal govrnment or ones who have been close to Carter in the past.

But the Carter sweep of Maryland's top political hierarchy left the Kennedy supporters with few officials from whom to garner endorsements.

Along with Schaefer and Gilchrist, those who will join Hughes at the Thursday press conference include former acting governor Blair Lee III, state Senate President James Clark, Montgomery County Council Chairman Neal Potter and Prince George's Councilman Francis B. Francois, who is also president of the National Association of Counties.

Carter began lobbying for support from Maryland politicians last October with invitations to an unofficial kick-off dinner for his reelection campaign at the Hyatt Regency Hotel in Washington. Those invited were told in mailgrams that their presence at the dinner would be the equivalent of an endorsement of the president.

The Carter forces failed to snare Hughes for that gathering. The governor said he had a previous engagement scheduled in New York, and continued to play coy later that month when he was questioned about his presidential preferences.

But last month, while still sticking to his "no-endorsement-yet" comments, Hughes made his intentions clear after attending a meeting of key Carter supporters.

"Will you be attending any meetings for Ted Kennedy in the near future?" he was asked at a press conference.

"I don't think so; I haven't been asked," he replied.

"If you are, will you go?"

"No. I don't think I will," said the governor, with a hint of a smile.