William Lucas, chief executive of Michigan's largest county and one of the state's most popular black Democrats, announced as expected yesterday that he will become a Republican, a step that could lead to a 1986 gubernatorial bid.

The switch represented a coup for national Republican leaders, who have launched a drive to recruit 100,000 disenchanted Democratic voters in the next 100 days.

Lucas, a former FBI agent, said the Democratic Party has come to be "controlled by narrow special interests striving only to achieve their own personal goals."

At a Detroit news conference, the Wayne County executive said blacks have "faithfully delivered all of our votes to one party, just to be taken for granted. Some of us believed that only one party represented the poor, only to find its proposed solutions have kept us poor and stripped many of us of our dignity."

"The Republican Party needs a national black leader," said Lee Atwater, a former White House political strategist who met with Lucas Tuesday. "For the first time we have our national black leader in Bill Lucas."

Lucas was elected Wayne County executive in 1982 with 77 percent of the vote and has earned a reputation as a hard-nosed budget-cutter. He had served 14 years as Wayne County sheriff, the first black to hold that job.

Wayne County includes predominantly black Detroit, and the county executive post is considered one of the most powerful in the state.

Lucas has been touted as a future GOP gubernatorial candidate by Rep. Guy Vander Jagt (R-Mich.) and other state party leaders. Yesterday he sidestepped questions about challenging Gov. James J. Blanchard (D) in 1986 in an attempt to become the nation's first black governor since Reconstruction.

"It would be presumptuous of me to believe that as a new member of the party I could step to the front of the line and be a nominee," he said. "That would have to be something I have earned."

Detroit Mayor Coleman Young, a Democrat who has feuded with Lucas, has compared Lucas' chances of being elected Michigan governor to those of the Rev. Jesse L. Jackson winning the 1984 Democratic presidential nomination.

"Far be it from me to dispute anything he Young says," Lucas said yesterday. "But even he would have to admit he can be wrong."

Lucas had been courted by the White House. Vice President Bush said Lucas' decision was "very, very good news for the Republican Party and the state of Michigan."

State Democratic Party Chairman Richard Weiner called the switch "the ultimate in opportunism." He said "the big deal" Republicans were making of the defection show their difficulty in finding someone to challenge Blanchard