On the same Alabama Capitol steps where he vowed "segregation forever" at his first inauguration 20 years ago, George C. Wallace today completed his political comeback by taking the oath of office for a fourth term as governor, a born-again populist pledging "mercy and justice" for all.

It was a cold Alabama day rich in southern irony, a stark contrast to Wallace's defiance two decades ago, when he invoked racial furies from the spot where Jefferson Davis became commander-in-chief of the Confederacy.

But as he became the first man ever to gain the Alabama governorship a fourth time, although non-consecutively, Wallace moved down the steps away from the bronze star embedded in the marble floor where Davis took his oath, symbolically closer to blacks in the crowd who elected him.

He placed his hands on a red family Bible and preached a doctrine of jobs and the survival of the middle class, the same themes he successfully evoked in an effort to shed his segregationist image in a bitter campaign. The band played "Dixie."

Whereas 20 years ago Wallace was defying the anti-segregation orders of the federal government, today he called for government aid to the poor and unemployed.

"For God's sake, let us hear the sighs of the hungry and the cold among us," said Wallace, 63, half deaf and confined to a wheelchair from a 1972 assassination attempt. "No one can be rich, as long as there are those among us who are hungry. Any nation that forgets its poor will lose its soul."

From his wheelchair he vilified greedy bankers and heartless bureaucrats on behalf of poor farmers and a working class in a state suffering 15.3 percent unemployment, among the worst in the nation. He has become a high-tech populist who once embraced states' rights and labeled one federal judge a "lowdown, carpetbaggin', scalawagin', integratin', pool-mixing, race-mixing liar."

Already what some political analysts are calling a "new" George Wallace has appointed two blacks to key cabinet posts--one the head of the state welfare department, the other to oversee revenue collections--and used his influence to have four black legislators appointed committee chairmen, more blacks than any governor has ever appointed to such positions.

"It looks like he's a changed person, but black people haven't completely pardoned Gov. Wallace yet," said state Rep. Alvin Lee Holmes, vice chairman of the state's largest black political organization, the Alabama Democratic Conference. "He's still on parole. Only time and his behavior will dictate his pardon."

There were no marching bands or parades, on Wallace's orders, as a reminder that times are austere. Wallace plugged for U.S. Agriculture Department surplus commodities.

"There is nothing wrong with the federal government maintaining surplus food supplies," said Wallace. "But neither is there anything proper about food piling up in America while people go hungry."

A largely blue-collar crowd of 2,000 farmers, factory workers, elderly retirees and small businessmen--hard times and hope etched on their faces--hardly rivaled the 100,000 people who flocked to see a fiery Wallace debut as a national symbol of segregation here in 1963.

"Let us rise to the call of freedom-loving blood that is in us and send our answer to the tyranny that clanks its chains upon the South," he railed two decades ago as 174 high school bands, 94 floats and 21 drill teams paraded by. "In the name of the greatest people that ever trod the earth, I draw the line in dust and toss the gauntlet before the feet of tyranny and I say, 'Segregation now, segregation tomorrow, segregation forever.' "

"That didn't mean a thing," shrugged R. Pierce, 75, a retired black school principal who sported a "Wallace crony" button here today. "White folks had to say that stuff to get elected back then."

Wallace inherits a state budget in shambles. General fund revenue, already cut 15 percent by former governor Fob James, is down 8 percent from last year as tax collections decline. Alabama owes Washington $82 million borrowed to prop up a bankrupt unemployment fund, and state budget officials are grim about 1983.

Still, if Wallace "does nothing else," said one white cotton broker, "at least he offers the people hope."