For decades, the South was the first among equals in the U.S. Senate, where figures such as Lyndon Johnson, Richard Russell, James Eastland, Harry Byrd and Sam Ervin were the personification of power, dominating issues ranging from civil rights to fiscal policy and national defense. Now, with the Democrats in serious contention to regain control of the Senate in this fall's elections, it appears that the South could rise again after six years in the shadow of a largely nonsouthern Republican hierarchy in the chamber.

The legendary barons of the Old South are mostly gone, replaced by a newer breed of younger, more moderate southerners who are becoming both more prominent and more aggressive within Senate Democratic ranks. They and their more senior southern colleagues are in line to take over half the Senate's major committees, including its key fiscal panels, if the Democrats gain at least four seats in the November voting.

Moreover, it is from this new group -- in the person of Sen. J. Bennett Johnston (D-La.) -- that Senate Minority Leader Robert C. Byrd (D-W.Va.) is expected to be challenged for the job of majority leader. Johnston is probably an underdog, but it is seen as significant that the major bid for change in the party is coming from its resurgent, newly activist southern wing.

Return of the Democrats to control of the Senate would shift its focus from the West and Midwest, which have produced most of the current cadre of Republican leaders, including Majority Leader Robert J. Dole (R-Kan.) and Majority Whip Alan K. Simpson (R-Wyo.), to the southern and border states that have traditionally contributed heavily to leadership ranks during Democratic regimes.

For a variety of reasons, southerners probably would be in an even stronger position after the Democrats regained control than they were before the party lost it, partly although not entirely because of the thinning of senior northern Democratic ranks over the past decade.

The new southern Democratic chairmen would include such old faces as John C. Stennis (Miss.) at the helm of the Appropriations Committee. But more typical would be Lawton Chiles (Fla.) on the Budget Committee, Lloyd Bentsen (Tex.) on Finance, Sam Nunn (Ga.) on Armed Services, Ernest F. Hollings (S.C.) on Commerce and Johnston on Energy and Natural Resources if he does not become majority leader.

Among the nonsouthern chairmen would be Claiborne Pell (R.I.) on Foreign Relations, Edward M. Kennedy (Mass.) on Labor and Human Resources, Joseph R. Biden Jr. (Del.) on Judiciary, Patrick J. Leahy (Vt.) on Agriculture, William Proxmire (Wis.) on Banking, Quentin N. Burdick (N.D.) on Environment and Public Works and John Glenn (Ohio) on Governmental Affairs. If Kennedy claimed his seniority rights to the chairmanship of Judiciary, Howard M. Metzenbaum (Ohio) would be chairman of Labor and Human Resources.

In cases where fiscal committee chairmanships would fall into southern hands, relatively little change in policy could be expected, because the current Republican chairmen, like the Democratic chairmen-in-waiting, tend to come from the moderate wing of their party.

Greater change could be expected on committees such as Judiciary, where Biden would succeed Strom Thurmond (R-S.C.), and Labor and Human Resources, where Kennedy or Metzenbaum would succeed Orrin G. Hatch (R-Utah). Both Thurmond and Hatch come from the GOP's conservative wing.

The increasing power and seniority of the new southern faces, in some cases relatively unknown ones, stems in part from the heavy toll taken in the ranks of senior northern Democratic liberals in the elections of 1978 and 1980, especially the Republican landslide that resulted in GOP takeover of the Senate in 1980.

It also reflects changes in the South itself. Death, defeat and retirement have all but stripped the Senate of the old barons, leaving only Stennis after the retirement of Russell B. Long (D-La.) at the end of this year. Black voting power, two-party competition and dramatic changes in the economy and demography of the region have combined to produce a different breed of southern senator.

These southerners tend toward pragmatism and away from flamboyance. "We're not only not ideological; we're anti-ideological," says Johnston.

In many ways, Chiles, a 56-year-old lawyer from central Florida who was elected to a third term in 1982, is typical of the breed. He has a slow drawl, a touch of populism, deep religious convictions and a lot of persistence. A longtime advocate of strengthening the military, he nonetheless led the fight two years ago to curtail production of the MX missile.

As ranking Democrat on the Budget Committee, he has worked closely with committee Chairman Pete V. Domenici (R-N.M.), and this year produced a bipartisan budget jointly with Domenici.

Claiming the Senate Democrats needed more vigorous leadership, he challenged Byrd two years ago and lost, but paved the way for a potentially stronger challenge this year. He has been active in efforts to pull the national Democratic party more toward the right.

Like Chiles, most Democrats who survive in the South tend to be more conservative than their northern counterparts, although, having been elected with black votes, they vote for civil rights legislation that their predecessors used to filibuster against. They tend to be pro-defense but also have enlisted in the fights to curb what they regard as excesses on the part of the Pentagon. Some have streaks of rural populism, but most would be comfortable in corporate board rooms.

They tended to take a back seat in national Democratic politics until recently, especially after President Reagan's 1984 reelection. With Republicans breathing down their necks back home, they have taken an active role in trying to recast the national party along what they regard as more "centrist" lines. Their control of such committees as Finance, Budget and Armed Services would give them influence in major areas where they are trying to change the party's image to one that is more attentive to fiscal restraint and defense needs.