Presidential intimates said yesterday that if President Reagan decides to seek another term, Republican political strategist Stuart K. Spencer is likely to assume a key role in the campaign, despite registering recently as a lobbyist for South Africa.

"Since the president hasn't decided whether he will run again, he can hardly be deciding on who will be his campaign manager," said deputy chief of staff Michael K. Deaver, who worked closely with Spencer in the 1980 campaign. "But Stu hasn't been eliminated in any way. The president has high regard for him."

Another source close to the Reagans said that "Spencer will certainly be aboard for 1984" if Reagan runs.

He pointed out that Spencer has continued to consult with Reagan. Spencer was invited to Palm Springs when Reagan vacationed there last New Year's holiday and spent two hours talking politics with the president.

Spencer was a key strategist for Reagan in the 1980 campaign, joining at the request of Reagan and his wife, Nancy, when the campaign was in a particularly rocky period. Reagan often turns in time of difficulty to aides of long standing, and Spencer helped manage Reagan's first campaign for governor of California in 1966.

Spencer told White House officials recently that he registered with the Justice Department to represent the government of South Africa.

But the source close to the Reagans compared Spencer's representation of South Africa with attorney Edward Bennett Williams' representing clients in criminal cases while serving as an official of the Democratic National Committee.

"This is just one of Spencer's many clients," the presidential intimate said.

The public relations firm of Deaver & Hannaford represented Taiwan and a group of Guatemalan businessmen as clients in 1980. However, Deaver took a leave of absence from the firm during the campaign and later sold his interest in it to Peter Hannaford before joining the Reagan administration.

However, the government of South Africa is viewed as a symbol of racist oppression by many in this country and throughout the world and Spencer's affiliation with it could be troublesome for Reagan in light of his already low political standing among black voters.

Spencer, in a telephone interview from California, said that he had registered as a lobbyist for South Africa in February and had told the White House beforehand. He said that his activity was limited to assisting in the formation of a government in Southwest Africa, which he stressed is "85 percent black."

"I knew when I did this that I would take some heat," Spencer said. "As far as I'm concerned, this doesn't take me out of any future Reagan campaign. If they feel different in the White House, that's up to them."

Presidential pollster Richard B. Wirthlin, who some had speculated might run a 1984 reelection campaign, said yesterday he has "no interest" in the post, but would like duties similar to his 1980 job in polling and strategy. Wirthlin said "no one in the White House" had discussed with him the possibility of running the 1984 campaign.

Even though Reagan says he has not decided whether to seek reelection, he has sent deliberate signals recently that he is thinking about it.

This week, he kicked off a series of luncheons for 1980 campaign supporters from outside the capital. In addition to lunch at the White House, the 60 Reagan backers at the first one were briefed by White House political affairs director Ed Rollins and Republican National Committee Chairman Frank Fahrenkopf, who said the event was "another sign that's made me extremely hopeful" of a Reagan reelection bid.