Bill Brock, who was named secretary of labor yesterday by President Reagan, grew up as William Emerson Brock III, the scion of a wealthy Lookout Mountain, Tenn., candy manufacturing family, which never had a union in its plants.

But one of his first calls yesterday went to his "old friend" Lane Kirkland, president of the AFL-CIO, who had broken off communications with Brock's predecessor, Raymond J. Donovan, for four years.

The flexibility and openness that prompted Kirkland to say "we look forward to a new and constructive relationship with the Labor Department" is characteristic of Brock's political saga -- and the reason some politicians viewed his choice as a significant move in the Republican Party's drive to displace the Democrats as the country's majority party.

"It's a great opportunity for Ronald Reagan and the Republican Party," said Charles Black, a political consultant who worked for Brock during his 1977-1980 stint as chairman of the Republican National Committee. "With the good political position the president has with rank-and-file workers, if Bill can expand our communication with the labor leadership and maybe find some areas where we can cooperate on issues, there is really a chance for a breakthrough."

Sen. James R. Sasser (D-Tenn.), who ended Brock's Senate career after one term when he defeated him in 1976, was skeptical. Recalling the "united and energetic opposition" of Tennessee and national unions to Brock in the 1976 race, Sasser predicted that Brock "will have a difficult time building bridges" to organized labor.

Former Democratic National Chairman Robert S. Strauss, who has worked with Brock on trade matters, called him "one of the most talented, gifted and able people in the administration," but said he thought that Reagan's economic and labor policies would neutralize any political threat that Brock's appointment might pose to the Democrats.

The appointment drew praise, however, from the traditionally Republican Teamsters to the staunchly pro-Democratic American Federation of State, County and Municipal Employes. A United Auto Workers official, who asked not to be named, said that during Brock's last four years as the administration's special trade representative, he "has always been accessible . . . always willing to hear us out." Another union official said when the normal channel of communication through Donovan was shut down, Brock, Secretary of State George P. Shultz and Vice President Bush "were the guys we could still talk to."

When Kirkland was asked by reporters on Tuesday what advice he had for the new Democratic National Chairman, Paul G. Kirk Jr., he urged him to emulate "the most successful political operators" he had seen at the helm of the Republican Party -- Brock and the late Ray C. Bliss.

At 54, Brock is a very different politician from the bright, brash and relentlessly well-organized young Washington and Lee graduate and tyro business executive who cut his political teeth in the brutal infighting of the National Young Republican Federation. An acquaintance -- and rival -- from those days said, "I'd say that his range of vision went all the way from right to ultra-right back then."

Brock maintained that reputation after winning a traditionally Democratic Chattanooga seat in the House of Representatives in 1962 and then moving up to the Senate in 1970, by defeating Sen. Albert Gore (D) in a campaign in which school prayer, busing and the Vietnam war were major issues on Brock's side.

As a senator, Brock worked effectively to reform the budget process and make other institutional changes, but maintained a conservative record that earned him only a 14 percent "approval" rating from the AFL-CIO.

It was as chairman of the Republican National Committee that Brock really developed the reputation for reaching out to nontraditional constituencies, including labor, that made him a natural choice as Donovan's successor.

The same post-Watergate election that cost Brock his Senate seat had seen Republicans lose the White House, and Brock argued insistently that Republicans "have to stop talking just to each other." He moved aggressively to establish ties to blacks, Hispanics, intellectuals and union people, who had voted overwhelmingly against Republican candidates.

The same impulse that brought the Rev. Jesse L. Jackson as a Brock-invited speaker to the Republican National Committee sent Brock himself to a convention of the liberal and politically active Communications Workers of America in the spring of 1978.

Acknowledging that the GOP was partially responsible for a "divorcement" from workers' concerns, he said he was "trying to end years of neglect," and invited unionists to help reshape the GOP. "The line is short," he said.

That same year, Brock mounted the first party effort to sell across-the-board tax reduction as a way of expanding the economy and creating new jobs -- a precursor of the 1981 Reagan program.

But Brock, who had defeated Reagan's choice for the party chairmanship in 1977, was in constant hot water with the prospective president, especially when he refused to put party money into a Reagan tour aimed at defeating the Panama Canal treaty.

When Reagan won the GOP nomination, there was serious talk among his strategists of dumping Brock, but he was allowed to remain in the chairmanship while others ran the Reagan campaign. Despite the sweeping GOP gains of 1980, Brock was passed over for all the traditional Cabinet posts and was given the Special Trade Representative post, which carries Cabinet rank, almost as an afterthought.

It measures a change in Reagan's view as much as in Brock's reputation that the man who was almost dumped from the party chairmanship has now been given one of the most sensitive political posts in the administration.