Last year, William Lucas, chief executive of Michigan's most populous county, was the biggest superstar in the Republican Party's Year of the Switcher, the Great Black Hope of his adopted party.

President Reagan invited him to the White House. Vice President Bush proclaimed the Wayne County executive's switch from the Democratic Party "very, very good news for the Republican Party." Michigan GOP leaders appeared ready to award Lucas the gubernatorial nomination almost by acclamation.

But 1986 has been an unexpectedly rough year for Lucas, seeking to become the nation's first elected black governor, and other prominent former Democrats attempting to advance in their new party.

Former congressman Kent R. Hance, another important Republican catch, finished a weak third in the Texas gubernatorial primary earlier this month. Tampa Mayor Robert Martinez, attempting to become the first Hispanic governor of Florida, is locked in a surprisingly rough and tumble GOP primary.

And here, many say they think that Lucas has been replaced by Richard Chrysler, a multimillionaire businessman, as front-runner for the GOP gubernatorial nomination, much to the chagrin of party leaders who think Lucas would be a stronger challenger to Gov. James J. Blanchard (D) next fall.

"It seems to me he is in still water," said Republican national committeeman Peter Secchia. "Lucas is limping along, and Chrysler is charging ahead full steam."

The party switchers insist that they have no regrets about joining the GOP, but their experiences hold warnings for other Democrats contemplating such a move. "Republican activists are resentful at worst and indifferent at best to someone who switches parties," concedes an architect of "Operation Open Door," designed to lure disaffected Democrats into the Republican fold.

Lucas, a former FBI agent, is a good case in point. Welcomed into Republican ranks a year ago last week, he became an immediate symbol of the party's hope for a breakthrough with black voters.

But since announcing his candidacy for governor, Lucas has been besieged with image-tarnishing controversies over conflict-of-interest charges in his county office, high-handed activities of his former chief of staff, financing of a gubernatorial exploratory effort and, more recently, the default on a $5,000 college loan to his wife.

"If the party leadership would have known him for 30 years, these things would have rolled off like water off a duck's back," Secchia said. "But Lucas is a newcomer. So you have to hold back instead of jumping to his defense. The first question you ask is, 'Do we really know this guy?' "

The charges put Lucas -- seen earlier as a different kind of black politician, a fiscal conservative not afraid to take on labor unions or Detroit's Democratic mayor, Coleman Young -- on the defensive.

Lucas "has become a mere mortal," said state House GOP leader Mike Busch. "There's a lot of prejudice in this state. You hear a lot of people saying, 'Maybe he's just another black politician from Detroit.' "

There also are lingering resentments about Lucas' quick rise in the party. "We all welcomed the conversion to our church. But I'm not sure we wanted him to lead the choir the first day," said L. Brooks Patterson, cochair of Bush's state presidential effort.

In Florida, Martinez, who switched parties in 1983, encountered a more open form of the same phenomenon when a group of party leaders organized the "ABM (Anybody But Martinez) Club."

His situation was complicated by a longstanding factional fight within the state Republican organization. Martinez, a popular two-term mayor, was recruited into the party by Tommy Thomas, Reagan's chief Florida operative, former state chair Henry B. Sayler and others long associated with the "coastal faction" of the state party.

The group has been warring for years with the "central Florida" faction, centered in the Orlando area. When Martinez's early momentum faltered, former congressman Louis Frey Jr., a longtime leader of the central faction, jumped into the race. "The criticism wasn't so much against Martinez, it was the people around him," said one party leader.

Martinez led Frey and another major challenger, Tom Gallagher, a state representative from the fashionable Coconut Grove section of Miami, in two statewide polls during April. The primary is Sept. 1, but the Frey camp already has broadcast a blistering set of radio ads, accusing Martinez of "managing Jimmy Carter's campaign" and leading a teachers' strike.

Martinez, a former teacher and labor negotiator, represented a local teachers' group during a 1978 strike. He said he supported Carter in the 1980 primary but "I was never in the Carter organization. I never raised any money. I never sat on any committee."

Martinez said he thinks the attacks will backfire on Frey. "I spoke at the Republican National Convention. I was chosen by the party for that high visibility position. When I switched parties, the president and vice president invited me to the White House. And of course, the crossover program is the president's program. So, in essence, he is speaking against the president's crossover program," Martinez said of Frey in an interview in Tampa. "That's not going to sell."

In Michigan, Lucas contends that he is still the front-runner, despite a late March poll conducted for Chrysler indicating otherwise. "It was unrealistic to think I could maintain the high profile I started with," Lucas said.

His biggest problem is money. Chrysler, who is not related to the famous automobile family, is financing his campaign largely himself. He has spent $1 million on advertising, relating the story of his rise from an assembly worker to owner of a custom auto manufacturing firm.

With a controversy involving the Michigan secretary of state holding up campaign matching funds, Lucas has canceled all public appearances to concentrate on fund-raising.

"Every poll indicates there is only one candidate who can beat the incumbent, and that's me," Lucas said in an interview. "I'm trying to make sure nobody can buy this election."