Secretary of State George P. Shultz once described William L. Ball III, the southerner who was then his chief liaison with Congress, as a man "who talks slow and thinks fast." In the two months since he has moved into the demanding job of White House director of congressional relations, the soft-talking Ball has ratified Shultz's assessment and become an important member of President Reagan's team during a period of intense conflict with Capitol Hill.

At 37, the rumpled, rotund Ball is a quiet persuader with a decade's experience in the Senate, where he became principal aide to two powerful members, Democrat Herman E. Talmadge of Georgia and Republican John G. Tower of Texas. As Talmadge's administrative assistant at 28, he was the youngest in his position in the Senate. Friends knew him as both fun-loving and hard-working, and said he kept his word.

"He's not a good old boy, but he tells good stories," says Kenneth M. Duberstein, who occupied Ball's job during the halcyon days of Reagan's first term and is now a successful lobbyist. "When he makes a commitment, you can take it to the bank."

Ball, who might be Duberstein with a southern accent, was in fact strongly recommended by Duberstein to White House chief of staff Donald T. Regan for the congressional relations directorship. It was in Ball's favor that he enjoyed the support of southern senators and had experience in foreign policy issues, which have provided the thorniest conflicts between Congress and the executive branch in the sixth year of Reagan's presidency.

Ball arrived at the White House at a time of particular tension in executive-congressional relations. While Reagan's popularity remains high on Capitol Hill and in the country, relations with the Democratic-controlled House have been strained by disputes on defense spending, deficit reduction and tax reform, and especially by the emotional debate on aid to the Nicaraguan counterrevolutionaries, or contras.

Since he succeeded M.B. Oglesby on Feb. 7, Ball has been battling in the halls of Congress to provide military aid for the anti-Sandinista rebels fighting in the jungles of Central America. The administration lost in the House, won in the Senate and is now caught in a parliamentary quagmire, ostensibly en route to a second House vote. Such veteran Democrats as Rep. Ed Jenkins (Ga.), who calls Ball "a class act," say the administration would have lost the first House vote by more except for Ball's efforts.

Ball is little known in Washington outside his circle of congressional friends. The only town in which the family name is a household word is Spartanburg, S.C., where Ball's father, "Billy Ball Jr.," is a retired Baptist minister. Ball calls himself "a preacher's kid," and he went from Georgia Tech and six years of naval service into politics by means of what might be called "the preacher's kids' networks." He was hired by Tower, says Ball, in part because both the senator and his administrative assistant were also children of ministers.

"I think that is what probably sparked my interest in politics," says Ball. "Preachers in many ways have to be practitioners of the political arts in dealing with their constituencies, their congregations. They're responsible for tending to a flock that ranges from A to Z. They have to raise money for their church, deal with problems both large and small, real and imagined, and in many ways do things that politicians are called upon to do every day."

Ball started out with the ambition of running for office himself, but abandoned this idea, he says, after he saw "what it is politicians have to go through." In Ball's case the experience was particularly searing because he became Talmadge's administrative assistant just before the veteran Georgia senator underwent an ethics investigation that resulted in a Senate denunciation and Talmadge's defeat in 1984. "Will's work as Talmadge's A.A. was exemplary," says Rep. John S. McCain III (R-Ariz.), a friend from Navy days. "Talmadge was in deep political trouble and personal trouble as well. There all these stories about money in his pockets. Some people think that Will during this time did the job of senator to a large degree."

Ball emerged from this experience with respect for Talmadge, whom he sees as largely a victim of alcoholism. "It was a tough time going through that investigation, but the man was able to solve his personal problems. He addressed his alcoholism head-on after years and years of glossing over that problem and having people around him who helped him gloss over it. He took it on squarely and licked it, and to this day hasn't had another drink."

Ball, a Republican who had worked for Tower as a legislative assistant before taking the job with Talmadge, returned to the Texas senator's employ after Talmadge's "When he makes a commitment, you can take it to the bank." -- Kenneth M. Duberstein, former White House director of congressional relations defeat. His experience on both sides of the aisle gave him what might be called a fading southern appreciation for bipartisanship, especially on national security issues. It is a useful perspective in an administration that is struggling to restore what Reagan likes to refer to as "a bipartisan consensus" on foreign policy at the same time that some White House spokesmen are sharpening partisan attacks against Democrats who oppose Reagan's Central America policy.

Ball has close friends among Democrats on Capitol Hill, including Jenkins and Mickey Leland (Tex.), chairman of the Congressional Black Caucus. During the first contra aid fight, Ball even called Leland and said he needed a couple of votes from the Black Caucus. He didn't get them, but he took a ribbing from Leland, who regards Ball as "warm and compassionate, not like some of these cold and calculating people" within the administration.

Leland suggested to Ball that the president might win some support for contra aid among blacks by supporting the African National Congress, the South African rebel group. As Leland remembers it, Ball replied, "Nothing is off the table." But Ball is also praised by moderates such as Sen. Daniel J. Evans (R-Wash.) and conservatives such as McCain, who says Ball has helped repair the damage with GOP members caused by "White House high-handedness" on the tax issue last year.

The strains that mark White House relations with Congress have also produced fissures within the administration between those who call for preserving the ideological purity of Reagan's agenda and those who believe that it is necessary to sideline some of the agenda to save most of it. It is a muted version of the debate that flourished in the first term, when rival power centers operated in the Reagan White House.

The hardliners, who think the president was damaged by even the mild compromises he made on contra aid, include White House communications director Patrick J. Buchanan and the influential director of central intelligence, William J. Casey. The "realists," whom Duberstein sees as the equivalent of the first term "pragmatists," include political assistant Mitchell E. Daniels Jr., chief Regan deputy Dennis Thomas and Donald R. Fortier, the assistant deputy for national security affairs. What they have in common is a background on Capitol Hill.

It has been a sign of Reagan's skill as a politician, since he was first elected governor of California in 1966, that he has defined his agenda broadly and allowed subordinates to achieve as much of it as they could. Both in Sacramento and in Washington, Reagan's offices of legislative affairs have been directed by skilled compromisers who knew the limitations of their own position and learned the weaknesses of the other side.

Ball, the first southerner in this crowd, fits the mold. A conservative who thinks in terms of national defense, Will Ball is nonetheless a practical man who would prefer, in the old phrase, "a bill to an issue," and bipartisan success on a key vote to an extra Republican seat in the next election.

"We're in the business of getting votes for the president's program," he says. "When they tally votes up on the board, they all count the same."