Lyn Nofziger, the veteran Reagan political operative who last week joined a conservative rebellion over the $98.9 billion tax increase, returned to the presidential fold yesterday and took charge of a White House offensive to sell the legislation to the American people.

Five days after he and two dozen other conservatives had promised to fight the tax increase, now in a House-Senate conference, Nofziger was at the White House describing to reporters his plans for a 10-day public relations blitz on behalf of the bill that may include a nationally televised appeal by President Reagan.

"I was opposed to this thing, but they finally done explained it to me," Nofziger said.

He got the explanation from an unhappy Reagan in the Oval Office last Thursday morning, Nofziger said in an interview yesterday. "There was some indication he wasn't real happy" with opposition to the tax bill by Nofziger, the former White House political director, and Martin Anderson, the former White House domestic affairs adviser.

Nofziger, sporting a new Mickey Mouse tie sent by a friend who heard of his most recent troubles, recalled that the president had complained to him: "I would have thought you would have asked me first" before joining the conservative revolt.

But he didn't.

"Pure stupidity," he sighed yesterday, then added: "At that particular time I had not met with people over here at the White House . I had thought they the conservatives were right. I'm like a woman--I changed my mind."

Clearly pleased by Nofziger's sudden enlightenment because it seriously undercuts the conservatives' mutiny, White House officials were lining up reporters for interviews yesterday to highlight his conversion.

Anderson, the former adviser who was also summoned last week to the Oval Office, had no comment yesterday when asked if he would join Nofziger. Deputy White House press secretary Larry Speakes predicted Anderson "will come along."

Also yesterday, Reagan announced that he would curtail plans to attend a fund-raiser in Los Angeles tomorrow for California GOP gubernatorial candidate George Deukmejian.

However, he will attend centennial ceremonies in Billings, Mont., tomorrow and return immediately to Washington. Last week, he postponed his planned California vacation to remain here and push for the tax and spending legislation.

Nofziger, who resigned from his White House post last year to return to private political consulting, will serve as an unpaid consultant, working from a White House office and using funds from the Republican National Committee, he said.

He sat in yesterday as Reagan lobbied another group of a dozen House Republicans for the legislation. Later, Nofziger told reporters he hopes Reagan will make a nationally televised speech for the bill, as congressional leaders have urged. Reagan is "leaning toward it," he added.

Nofziger said the White House wanted to put before the American public the "positive" aspects of the tax bill, that it preserves the tax cuts and tax indexing of last year, that it will bring interest rates down and help bring federal deficits under control.

Nofziger said the White House would attempt to win over rebellious House Republicans, led by Rep. Jack Kemp (N.Y.), by taking the case for the tax bill to their constituents.

"I don't want to charge Jack with sabotage," Nofziger responded to queries about Kemp. "I think he's sincere, but maybe he is a little fanatical on this. He is in spite of himself, hurting the president and hurting the presidency."

"Jack is doing what he thinks is best. I think in the process he is helping divide the party, which I think for his own long-term best interests is not the best thing to do." Other White House aides privately accused Kemp of pursuing his own presidential ambitions at Reagan's expense. Kemp maintains he is only being consistent with his past tax-cutting crusade.

Rep. Manuel Lujan Jr. (R-N.M.), one of those who participated in yesterday's session with Reagan, said the president made the argument that "80 percent" of the tax bill "is closing loopholes." Office of Management and Budget Director David A. Stockman said that while the bill will raise about $100 billion over three years, "$30 billion is collecting taxes that are owed now, not new taxes, and another $30 billion is closing loopholes that shouldn't have been there in the first place . . . ."

Reagan also will resume his five-minute weekly radio broadcasts this fall, White House officials said. They will run for 10 to 13 weeks, perhaps indefinitely, and he might take questions from listeners. He did 10 live weekly broadcasts beginning last April 3.

House-Senate conferees on the tax bill, meanwhile, worked all day on a series of proposed compromises on spending cuts involving Medicare, Medicaid and welfare, and on a package of tax compliance provisions, but made few major decisions and recessed overnight. Senate Finance Committee Chairman Robert J. Dole (R-Kan.) said he hopes to finish the bill tonight.