When Jimmy Carter went to the mountain top in July to assess his presidential performance, he came down echoing the quiet, reflective counsel of his close friend and backer, South Carolina Gov. Richard W. Riley.

Riley, 46, is the novice Southern governor who told Carter during the soul-searching summit at Camp David: "Mr. President, you are not leading this nation. You are just managing the government." Wiley's candid advice hit so close to the mark that Carter quoted it on television when he told the nation his administration would henceforth be more assertive in tone.

Carter is not the only one seeking -- and heeding -- the advice of South Carolina's unassuming yet forceful Democratic governor.

This month, Congress and the Nuclear Regulatory Commission took notice when Riley went to Washington seeking a national solution to the problem of low-level nuclear waste disposal.

By significantly reducing the amount of waste allowed into his state, Riley has forced the federal government and other states to consider a problem with which South Carolina was grappling alone as storage site for 85 percent of the nation's nuclear garbage.

Riley left the capital -- two days, one congressional hearing and a major television appearance later -- convinced that legislation he drafted setting up regional sites has garnered enough support to be the framework for the first national policy governing storage of low-level nuclear waste.

Back home, Riley alarmed state political leaders last winter when he announced his plan to strip lawmakers of their power to fill seats on the Public Service Commission, tradition al sinecures for former legislators. Through a lobbying blitz that made him a few enemies, Riley came out of the legislative session with a compromise bill that, starting in 1980, will turn over the process to a merit selection panel appointed by the governor and the legislature.

Riley's newest crusade will be regional. Last week, he assumed the chairmanship of the Southern Growth Policies Board for a one-year term.

Riley has promised to make the research and development board a major force in dealing with the federal government on matters affecting states, a task he says his experience as a Carter-appointed member of the congressional Advisory Commission on Intergovernmental Relations has prepared him for.

"I don't feel like the South is the stepchild of the nation. I think we're very much in the mainstream," Riley explained in a interview. "But I will certainly be eager to serve as a spokesman for the region, to advise congressmen from time to time of things that will particularly affect the South."

A native of Greenville, a textile-oriented city of South Carolina's mountainous upcountry, Riley at age 21 was striken with a crippling arthritic condition called rheumtoid spondylitis.

For 15 years he fought the pain in his spine, hips and shoulders, all the while establishing himself as a promising young attorney in his father's small Greenville law firm and never missed a day's work. By the time the arthritic condition stabilized, Riley had a permanently stooped back and an immobile neck that today give him a deceptively frail appearance.

But Riley's physical condition did not stand in his way when he sought election to the state legislature in 1962. He served in Columbia for 14 years, gaining a reputation as a legislative peacemaker who quietly championed controversial causes such as judicial reform and local government autonomy.

In hometown Greenville, meanwhile, he helped keep the peace when controversial issues forced their way to the top of the public agenda.

In 1970, when the city school system was faced with a court order demanding rapid desegregation, Riley went on a radio talk show to plead for calm and racial harmony.

He organized and led a citizen's group monitoring the desegregation effort, later praised as one of the speediest and least disruptive in the nation. And while many public figures were sending their children to all-white private schools, Riley kept his four children in public schools.

Though popular, Riley in 1975 announced he would not run for reelection to the state Senate but would instead work as a full-time volunteer for his friend Jimmy Carter, who was running for president. Riley managed the Carter effort in South Carolina, a move that he says may have hurt his own statewide political ambitions.

But the contacts he made during the Carter campaign helped Riley to overcome a tiny 3 percent recognition rating when he began a candidacy for the South Carolina governorship. In the 1978 primary, he overtook a popular lieutenant governor and veteran congressman, William Jennings Bryan Dorn, and won the general election easily over a former Republican congressman, Edward L. Young.

Riley's Carter ties have proved useful to a state whose leaders have rarely gotten on well with the White House.

Those who have watched Riley operate in Washington are convinced that his White House connections saved Fort Jackson, a major Army installation in Columbia, which earlier this year was on a Pentagon shutdown list. But San Tenenbaum, a steel company executive and longtime Democratic Party fund-raiser, says Riley has been careful "not to go to the president with a shopping list."

"Just because we want something in South Carolina doesn't mean he's going to ask for it. If it's what we need, if there's a real program that has success, then I think you'll see Riley going to bat for it and using all the influence he has. But to say, 'I got the bigger piece of pie because I knew Jimmy Carter,' that's not Dick Riley," Tenenbaum said.

Riley, who already has come out strongly for Carter's reelection, says his relationship with the president has simply helped him know whom to contact when he goes to Washington with a problem.

"It enables me to know who I'm talking to when I'm up there and to quickly get to the right person.And certainly I know as long as I'm fair with them, they'll be fair with me," he said.

The Carter-Riley friendship also has brought the governor quickly to the national limelight. When Vice-President Mondale went to China this year, for example, Riley went along as one of a handful of invited guests.

The governor's quickly cultivated national profile has led observers in South Carolina to wonder about his ambitions. But Riley dismisses all speculation by saying his only aim is to be "the best governor I can possibly be."

He is frequently mentioned as a possible candidate for the Senate seat of veteran Republican Strom Thurmond, who has announced he will not seek reelection in 1984. But that race would likely pit Riley against popular Third District Democratic Rep. Butler Derrick, with whom the governor has worked closely on state-federal matters.

Derrick, who admits he's interested in Thurmond's Senate seat, said he "cannot conceive of any situation where Dick Riley and I would be at odds.I think if it ever came to that, we would have to sit down and work out something. He's the most popular man in the state right now, and he would do well any place you would put him."

Other Riley watchers think he will seek a constitutional amendment this year allowing South Carolina governors to hold second four-year terms. Riley concedes he would "be interested in that, if at the end of four years I feel like I've done a good job and thought that I could move some things" by continuing in office.

But those who have worked with Riley doubt that his career will end in Columbia.

As Tenenbaum explained, "The people in Washington ought to keep their eyes on him. He's an unconventional political phenomenon who has the conventional political wisdom to know how to win. And in unconventional politics, Dick Riley is a comer."