In 1969, Maryland Gov. Marvin Mandel was looking for a way out of a ticklish problem. He needed an acceptable candidate to succeed him as head of Maryland's fragmented Democratic Party.

Mandel's solution, after months of searching, was to lend his powerful backing to Harry R. Hughes, an urbane, articulate, often seemingly boyish. Eastern Shore liberal who was then the Democratic floor leader of the State Senate. Hughes seemed compatible with Mandel, but posed no political threat to the governor, Hughes also had enough stature of his own to head off criticism that the new Democratic chief would be only a handpicked Mandel lackey.

Yesterday, Hughes, now 50, resigned in protest from another ticklish Mandel assignment. Since 1971, he had been the state's secretary of transportation, the first official to hold the controversial and powerful Maryland post.

The move struck some Maryland government insiders yesterday as uncharacteristic for Hughes, who has often been viewed as a moderator and conciliator of disputes. "I've never known Harry to be mercurial enough to resign a job in protest - not unless he's got somethinglined up," one Mandel intimate remarked yesterday.

Despite apparent abruptness of Hughes' resignation - over a controversy surrounding, a management contract construction of Baltimore's new subway system - Hughes himself handled the occasion with his usually calm demeanor. He said his decision was not sudden. "Frankly, I've been on the verge of it for some time," he told a news conference.

What he will do next is unclear. An aide said he might go back to practicing law, as he once did on the Eastern Shore. There was speculation he might join a consulting firm in the transportation field. Hughes himself did not rule out a possible race for governor or other state office.

Over the years, Hughes has displayed more talent as a government technician and compromiser than as a vote-getting politician.

He was defeated for Congress in 1964 by then-Rep. Rogers C.B. Morton, a popular Republican and Nixon Cabinet appointee. Hughes did not seek re-election to the State Senate in 1970 and when he accepted the job as secretary of transportation, he put an end to speculation at the time that he would run for Congress again.

In the General Assembly and the Department of Transportation. Hughes showed a keen grasp of financial issues troubling the state - one of the technical fields in which he proved most astute.

He was a chief author of Maryland's 1967 graduated income tax program, and he was for six years chairman of the Maryland Senate Finance Committee. As transportation secretary, he built his programs around a consolidated transportation "trust fund" - a key financing mechanism drawing revenue from the state's gasoline tax and other transit fees.

Born in Denton, on Maryland's rural and politically conservative Eastern Shore, Hughes hold a bachelor's degree from the University of Maryland and a law degree from George Washington University. He served in the Maryland legislature for 16 years - four as a delegate and 12 as a senator, including six as the Senate Democratic leader.

In his six years as transportation secretary, his aim was to maintain a "balanced" transit program - with even stress on highways, mass transit, Maryland's airport and Baltimore's port.