Alexandria Vice Mayor Margaret B. Inman, considered the Republicans' strongest candidate to win the 1988 mayoral election, announced yesterday she is resigning because she is getting married and moving to South Carolina.

Her decision surprised many of her supporters and colleagues, and wounds the Republican Party, which last year came within 215 votes of gaining their first majority on the council since Reconstruction.

As the top vote getter in that election, Inman was deemed the leader who might halt the Democrats' domination and even win the mayoral seat for her party in 1988.

A former aide to five-term Democratic mayor Charles E. Beatley, Inman was first elected to the council in 1979. She plans to marry Alexander B. Clarke, a retired officer of Sohio Petroleum Co., and a resident of Clover, S.C., on Saturday.

Asked why she is leaving politics at the height of her public career, she said, "I thought Mr. Clarke made me a better offer."

"I think she was deservedly very popular," said Mayor James P. Moran Jr., a Democrat. "I feel sadness at the prospect of her leaving, but I'm pleased at her happiness."

Republican and Democratic leaders said a special election is likely to be held this fall, but the date will not be set until Inman decides when she will leave. "I'm not in a hurry . . . . We're going on a honeymoon after the wedding, but I'll be back for the next meeting [June 10]," Inman said. She intends to move later this year, she said.

Inman, who has three children from a previous marriage, has been a champion of human services and environmental causes.

"She has brought in people who had not thought of voting Republican before," said Republican Party Chairwoman Jane Ring. "Alexandria is losing a tremendous citizen."

"Sometimes when these personal things occur, the first thing we think of is a special election," said Moran. "But we're . . . happy for her."