Maryland Secretary of State Lorraine M. Sheehan, a candidate for lieutenant governor who hopes to turn her courtship with gubernatorial aspirant Stephen H. Sachs into a full-fledged political marriage, plans to endorse Sachs' bid for the state's highest office on Monday, according to sources close to the two candidates.

Sheehan, who has strong political ties to Prince George's County Democrats, is scheduled to make her announcement from the steps of the county administration building in Upper Marlboro. It comes as Sachs, the state's attorney general, is winnowing his small field of potential running mates for the 1986 election. In Maryland, unlike states such as Virginia, candidates for governor and lieutenant governor must run together on a ticket.

One Sachs campaign organizer stressed that while the two had not agreed to run together, Sheehan's announcement on endorsing him could bode well for her eventual selection by Sachs. "To say a deal is done and signed is inaccurate, but it would be disingenuous to say they haven't had a lot of intense discussion about this," the campaign aide said.

Although Maryland's statewide elections are more than a year away, Sheehan's scheduled announcement struck some Democratic observers as significant because it marks the first in a long line of expected major endorsements in the already accelerating governor's race.

Neither Sachs nor Sheehan, both Democrats, would comment yesterday about her scheduled news conference Monday, but Sheehan said she would use the occasion to announce her favorite for governor.

The choice of a running mate is crucial to Sachs, 51, who to become governor must expand his electoral base in the Baltimore area by cultivating support in such suburban jurisdictions as Prince George's and Montgomery counties. That political predicament is shared to some degree by Sachs' probable rivals, Benjamin L. Cardin, the speaker of the Maryland House of Delegates, and Baltimore Mayor William Donald Schaefer.

In that regard, Sheehan could be a canny choice for Sachs, several prominent Democrats said yesterday. "It would be an exciting ticket," said Montgomery County Executive Charles W. Gilchrist. "They're articulate and energetic people, both of them."

Sheehan, 47, is a popular politician in Prince George's, which she represented for several years in the House of Delegates. First elected to the House in 1974, she established a strong rapport with Cardin and a reputation as an effective advocate for the handicapped, the mentally ill and the aged. Her ties to Prince George's and her philosophical links to Montgomery liberals could aid Sachs in those two counties, several Democrats said.

The Sachs campaign organizer noted that Sheehan's chances of becoming Sachs' running mate improved recently when Sachs ruled out Baltimore County Executive Donald P. Hutchinson. Hutchinson is contemplating a race against Republican Rep. Helen D. Bentley.

Moreover, party activists said, the choice of Sheehan as a running mate would give an electoral ticket a cachet similar to that of former presidential candidate Walter F. Mondale, who chose Geraldine Ferraro to run with him last year.

Maryland has never had a woman lieutenant governor, but Sheehan is not the first to seek the office.

Selecting Sheehan would mean "that in one stroke Sachs not only bolsters his position but makes an excellent choice besides," one Montgomery Democratic leader said.