Secretary of Labor William E. Brock will resign Thursday to become chairman of the presidential campaign of Sen. Robert J. Dole (R-Kan.), according to Labor Department and campaign sources.

Brock will bring to the Dole campaign a reputation as an organizer and strategist. Brock's roots are in the South, where Dole's organization is considered weakest.

In the 1960s, Brock helped build a strong Republican Party in his home state of Tennessee, which he has represented in the House and Senate. He served as chairman of the Republican National Committee from 1977 to 1981, helping to restore financial and political vitality to the GOP after the Watergate scandal.

Campaign sources said former representative Robert F. Ellsworth (R-Kan.), political director of Richard M. Nixon's 1968 presidential campaign, will give up the chairmanship of the Dole campaign, but remain in a position described as a "senior" or "important" adviser. Ellsworth encouraged Dole to persuade Brock to join the campaign, and the shift does not reflect internal dissension, the sources said.

When Dole ran for the Republican presidential nomination in 1980, he was sharply criticized for refusing to delegate power to campaign operatives and strategists, insisting instead that he review, and often veto, minor details of scheduling and other staff activities. That year, he received 607 votes -- less than 1 percent of the total -- in the New Hampshire primary.

The selection of someone with Brock's stature is a clear signal that authority will be delegated in the 1988 Dole campaign. Campaign sources made it clear that "Brock will be running the campaign. He will be in charge."

While Brock will run the campaign, day-to-day operations will continue to be directed by William Lacy, former political director of the RNC and political aide at the Reagan White House.

Brock has been serving as labor secretary since 1985. In that capacity, he has been credited with improving relations between organized labor and the Reagan administration. Brock served from 1981 to 1985 as U.S. special trade representative.

Brock becomes the second member of the Reagan Cabinet to leave to join the Dole campaign. Elizabeth Hanford Dole, the senator's wife, gave up her post as secretary of transportation to campaign for her husband.

Brock served one term in the Senate by defeating incumbent Sen. Albert Gore (D-Tenn.) in 1970. Gore's son and namesake is now seeking the Democratic presidential nomination.

Dole and Brock were colleagues in the House and Senate, where the two were known as deeply partisan GOP advocates. In addition, both Dole and Brock have served as RNC chairmen. Their terms were separated in part by the chairmanship of Vice President Bush, who is also running for the GOP nomination.

During his term at the RNC, Brock and then-presidential candidate Ronald Reagan did not always get along. Reagan supporters were particularly angered by Brock's refusal to put RNC monies into a Reagan tour designed to promote opposition to the Panama Canal treaties.