Howard County Executive J. Hugh Nichols, described by a former fellow Democrat as "a man on the move," registered as a Republican today as expected, turning his back on a party that spurned his gubernatorial ambitions and joining one that regards him as a leading contender for the nomination in 1986.

Before a crowd of 60 Maryland Republican Party members and well-wishers here, Nichols insisted that the switch was not "predicated on a race for governor." He said it would be "presumptuous" to be considered qualified to lead the party "30 seconds after joining it."

But close friends said it also would be out of character for Nichols, the son of an Alabama sharecropper, to pass up such an opportunity.

"He's a man on the move," said State Sen. James Clark Jr., a Howard County Democrat and political mentor of Nichols. "I wouldn't be surprised at all if he got into the race."

Republican leaders, meanwhile, gleefully speculated on the prospect of a Nichols campaign.

"If Hugh Nichols decides to run for governor, Maryland will be a targeted state. That means good financial support and good auxiliary support from the Republican National Committee," said state GOP Chairman Allan C. Levey.

The Howard County executive, who cannot seek reelection when his term expires next year, said today that he had decided to abandon the Democratic Party after a "very careful analysis" of his objectives as a public official.

"I feel good in knowing that I will no longer have to force my goals and my beliefs into shoes that don't fit," he said to applause. Nichols, who considers himself a fiscal conservative, said the goals of the Republican Party -- lower taxes, decentralized government and a free economy -- were more in keeping with his own.

"He's a very competent man. I am very sorry that he left the Democratic Party," said Baltimore Mayor William Donald Schaefer, one of several in the party considering running for governor.

Last March, Nichols was the first Democrat to officially file for the campaign. It was to have capped a 20-year political career that, like his life, started at the bottom.

His parents were tenant farmers in Alabama, and at 17 he dropped out of high school to support the family when his father was stricken by tuberculosis. Nichols joined the Army, took courses by mail, and eventually graduated Phi Beta Kappa from the University of Alabama with a degree in political science.

He moved here in 1957, worked as a public affairs director and consultant, earned a master's degree at American University and in 1966 was appointed to Howard County's charter board. Three years later he was elected to the County Council. He was appointed to fill a vacancy in the Maryland General Assembly in 1970 and was elected to two terms after that.

"He was a competent legislator; he did a fine job on the Appropriations Committee," said House Speaker Benjamin Cardin, who may be a Democratic candidate for governor himself.

In what was considered a long-shot campaign, Nichols beat an incumbent in 1978 to win a first term as county executive, while the rest of his slate lost. He was unopposed in 1982 in the Democratic primary and the general election.

Friends say Nichols has always believed in hard work, maintaining a positive attitude and taking advantage of opportunity. They say he was deeply disappointed when Democratic leaders were indifferent to his gubernatorial campaign.

"I'm a good salesman; It's hard for me to admit I couldn't sell them," Nichols said.

And so last month, Nichols dropped out of an already crowded Democratic field, which includes better-known and better-financed candidates such as Cardin and Maryland Attorney General Stephen H. Sachs.