It might be called the Twilight of the Bulls, this time of waning domination of the old Southern power brokers who for so long controlled the congressional process.

An exception to this new reality on Capitol Hill is that one of the last of the Deep South's political relics, Rep. Jamie L. Whitten (D-Miss.), 68, appears about to ascend a pinnacle of power.

By dint of his 19 terms in Congress, going back to 1941. Whitten is the odds-on favorite to become chairman of the House Appropriations Committee when the Democratic caucus votes next month on chairmanships.

If chosen by the caucus, Whitten would succeed Rep. George Mahon (D-Tex.), who chose not to seek reelection. Only Mahon has served longer on Appropriations than Whitten.

The prospect of Whitten's ascension has set consumer and environmental groups to fearing for the republic. It alarms some reform-minded House members, who see the conservative Mississippian as the wrong man for times requiring foresighted leadership and daring.

They would rather, for example, see the more moderate Rep. Edward P. Boland (D-Mass.), who ranked just behind Whitten in seniority, as chairman . But for now, there has been no visible draft-Boland movement.

Environmentalists feel he is enamored of wasteful pork-barrel water projects. and they loathe his belief in extensive use of farm chemicals. Nutritionists harrumph at his support of junk foods in the schools. Integrationists deplore his long-standing efforts to prevent busing to desegregate schools and others regard him as a race-baiter who only lately has grudgingly recognized that blacks are human beings.

But most agree on his legislative traits: Skillful. generally fair. always dogged. irascible and tough. possessed of a long memory. more courtly than not.

Fears about Whitten stem from his 28 years as chairman of the Appropriations subcommittee on agriculture.

Because of his tenure and the power inherent in Appropriations subcommittee chairmanships. Whitten is one of the nation's most influential figures on agriculture policy.

Secretaries of Agriculture come and go. but Whitten has been there for three decades watching over the shape of farm policy. His clout is such and his knowledge of the workings of the Department of Agriculture so broad that he had often been called the "shadow" secretary.

A variety of past and present federal farm programs-soil conservation. heavy subsidies for research. emphasis on pesticides-are a direct result of Whitten's insistence.

Likewise, crities say the decline of the small family farm. the growth of agribusiness. the ominous increase in crosion of vital topsoil are inevitable fallout from Whitten's influence.

The policy skirmishing reached one of its more pronounced peaks this year when Whitten and Bob Bergland. the secretary. clashed angrily over USDA's budget.

A central element in the clash was the amount and direction of the multimillion-dollar federal investment in research. Whitten over the years has strongly backed research by USDA itself and by the land-grant universities-production-oriented research that tends to bolster the fortunes of large farming enterprise.

Through that state-by-state distribution of money and the security it provides to a hefty federal bureau-cracy has evolved a national network of potent political backing-"a sort of Viet Cong research infrastructure." a Whitten-watcher calls it.

The Carter administration's intention was to redirect some $30 million of the research money to. in the words of Assistant Secretary M. Rupert Cutler. "new blood." to explore new areas of nutrition crop sciences and pest control.

Whitten. first in his subcommittee and then in the full Appropriations Committee. simply steamrollered the secretary. A furious Bergland said he "presented" the congressman's muscle-flexing.

"He wants to run the department through these agencies" of USDA. Bergland said in reference to the dispute over research and other activities. "It is a question of who is going to run the department-the heads of agencies or the secretary of agriculture."

Bergland won part of the battle when the Senate. to which he had appealed. took the administration's side and forced Whitten to accept some appropriations for outside research and some closings of USDA laboratories.

In a sense. Whitten was doing no more than other subcommittee chairman do in their own areas of interest. But Whitten because he knows USDA backward and forward and because he is so unrelentingly persistent-has elevated this to an art form.

An interesting aside in any report about Jamie Whitten is that many of his supporters are as reluctant as his crities, inside Congress and out, to be quoted about him by name.

The fear of budget retribution or misunderstanding and the power of the appropriations pursestrings run that deep. Even in the warrens of the USDA. said one high-level official. "the general attitude toward Jamie Whitten is one of fear. because of his power and his leverage."

One who doesn't mind being quoted, Russell Train former head of the Environmental Protection Agency. said that while he found Whitten "not sympathetic" to environmental programs. "generally, his bark was worse than his bite."

"If you had your choice. you might pick someone else to be chairman of the full committee," Train said. "I tend to think some people's concern about him as chairman is over exaggerated. If I had to guess, I would say it wouldn't make a lot of difference."

If Whitten, contemplating the committee chairmanship, has any notion of changing his style, he is not talking about it. He declined a request for an interview.

The biggest complaints against Whitten, ironically, were largely resolved in 1975. Public pressure and the rambunctious freshmen elected in 1974 threatened his control over consumer and environmental appropriations.

Before the party caucus could strip him of those powers-and, conceivably, his subcommittee chairmanship-Whitten stilled the critics by surrendering jurisdiction over the environment and consumer money.

For years, through the appropriations process, Whitten had attempted to stifle outlays for a number of environmental control efforts, including reduced use of pesticides, and for nutrition and consumer-protection work.

One controversial episode in his legislative career - of which his critics still are unforgiving-came when he wrote a book in 1966. "That We May Live" was intended to debunk Rachel Carson's "Silent Spring," which warned of dangers in excessive use of pesticides.

His thesis that pesticides are "an absolute necessity to our way of life" was undercut somewhat by disclosures that the book was conceived by and subsidized by pesticide industry executives angered by Carson.

Whitten has had time to look beyond his 1st District's borders because he is so politically safe there. Only this year, for the first time since 1962, was he forced to take his opposition seriously.

He wound up defeating a spirited Republican opponent, T. K. Moffett, with almost 70 percent of the vote. His last serious challenge was in 1962, when he ran against Rep. Frank Smith, a liberal Democrat whose district had been reapportioned into Whitten's.

It was a nasty campaign, with Whitten slurring Smith's progressive views on race and his ties with President Kennedy.

"That was the only time he played that public game on race," recalled a Mississippian who now works for the Carter administration. "He plied the standard of time, but he was always more invisible with the racial stuff than others. He has always represented the basic white establishment of the district-never the blacks or the poor whites."

This time around, with blacks now making up 37 percent of his district race was a silent but important factor. Moffett worked overtime to capture the black vote: Whitten responded by hiring a black for his congressional staff, billing himself as a congressman for ALL the people.

If the Democratic caucus votes as expected, Jamie Whitten next month will be Appropriations chairman for ALL the people. He will be watched closely. CAPTION: Picture, REP. JAMIE L. WHITTEN . . . odds-on favorite