Howard County Executive J. Hugh Nichols, whose gubernatorial ambitions were spurned by his fellow Democrats, has scheduled a press conference today to announce that he is switching to the Republican Party, according to Republican activists. It is a move that is widely seen by both parties as a prelude to a 1986 run for governor under the GOP banner.

Nichols, who has been courted by Maryland Republicans for several months, and more recently by the national GOP, has consistently refused to say whether he would switch parties since dropping out of the Democratic gubernatorial race last month. But many GOP officials and officeholders have been invited to attend the press conference in Ellicott City.

"He is going to switch," said one Republican officeholder yesterday, who asked not to be identified. "I was called and told to be there."

Nichols could not be reached for comment.

"We've done everything we could to make him know he would be welcome and comfortable in the Republican Party," said state GOP Chairman Allan Levey. "I'm certainly optimistic that's what he is going to do, but I don't know."

Nichols is not expected, however, to immediately declare his candidacy for the GOP nomination for governor, apparently preferring to wait until he is sure the party will make good on its pledges of support and financial help.

Within a party that still faces severe handicaps in capturing statewide offices, despite its recent successes in winning the state for President Reagan, the prospect of Nichols' candidacy has been greeted enthusiastically as a means of achieving instant credibility.

"If it happens, it's going to be a shot in the arm for the Republican Party," said national committeeman Richard Taylor.

Taylor, a self-described "staunch conservative," predicted that Nichols' moderate politics would appeal to Republican voters "pretty much across the board," despite his long service as a Democratic official. Nichols, 54, who is completing his second term as county executive, previously served in the state legislature, was an assistant budget secretary to then-Acting Gov. Blair Lee III, and was a member of the County Council.

During Nichols' seven years as county executive, the county has experienced strong growth and a steady increase in its tax base. Nichols often touts his management experience, and during his first term he oversaw a period of rapid expansion in capital projects.

Meanwhile, Howard County developers have repeatedly criticized the county's planning office under Nichols for what they say are inordinate delays in processing subdivision plans. And last year, Nichols' adminstration was rocked when auditors uncovered a $283,000 embezzlement scheme in the county's Office of Finance. An audit later found that management controls were too lax in the office.

Lacking a strong candidate from within their own ranks and facing a 3-to-1 Democratic voter registration advantage, Maryland Republicans turned their attentions to Nichols in earnest last month when he dropped out of the Democratic race after finding little support among that party's regulars.

With Nichols running for governor and U.S. Sen. Charles McC. Mathias Jr. running for reelection in 1986, Levey said the GOP would have a "dream ticket . . . that would help create a two-party system in Maryland."

Predictably, many Democrats feel that a quick fix is no substitute for the kind of long-term party building effort necessary to overcome their large registration advantage and virtual domination of local and state offices.

Said Joseph M. Coale III, who served as Gov. Harry Hughes' campaign manager in 1982, "Republicans just don't want to do the work. They don't want to get their fingernails dirty eating hard crabs."