When the Reagan-Bush campaign has wanted to raise a large amount of money, it has often pulled out one of its top stars: Treasury Secretary Donald T. Regan.

"He is probably second to the Reagan family and the vice president" as a fund-raising attraction, said John Buckley, deputy press secretary for Reagan-Bush '84. "He's probably the chief draw Republicans have. He is an enormous draw."

Regan attended two fund-raisers for the Reagan-Bush campaign and helped attract $350,000, Buckley said. For Victory '84, another Republican fund-raising group, Regan raised $150,000, Buckley added.

It is estimated that Regan also helped raise between $750,000 and $1 million for congressional and other Republican events, Buckley said.

At his appearances, Regan generally talks about the economic recovery, the success of Reaganomics and failure of the Democrats' policies of "tax and spend." Regan is also considered a good ad-libber who can speak without a text.

"He's very popular," Buckley said. "Republicans like his policies. He is just very well liked," particularly among the middle-income and upper-income Republican crowds.

One middle-to-upper-income Reagan supporter who doesn't like Regan's policies is Martin Feldstein, the former chairman of the Council of Economic Advisers who was in town this week pushing for a tax increase next year to help close the federal budget deficit.

President Reagan has said that he would seek a tax increase only as a last resort and Regan has adhered to that line, contending that economic growth and spending cuts will reduce the deficit.

When asked about Feldstein's remarks on the MacNeil-Lehrer Newshour this week, Regan replied, "Marty's advice wasn't accepted then, when he was in the administration, that's why he left, and it's not being accepted now. The president doesn't want tax increases."

Feldstein has maintained that he left the administration not because of economic policy squabbles with Regan and others but because he had been away from Harvard University for two years and had to return if he wanted to retain his position there.

Feldstein could not be reached for comment yesterday. COMINGS AND GOINGS. . .

Assuming Reagan wins reelection, it appears that Regan will continue in his job in the next administration, or at least until the tax reform push is completed. Officials said that so far, it doesn't appear that any of his top aides plan to leave either.

However, Thym S. Smith, deputy assistant secretary for public affairs, is leaving Nov. 15 to join G.D. Searle Corp. as vice president for public affairs of the Nutrasweet division. Smith was public affairs chief at the Internal Revenue Service from March 1982 to March 1984, and before that was press secretary for the National Republican Congressional Committee for three years. Smith has been in his present post for seven months.

Deputy U.S. Treasurer David Q. Bates will leave his job to practice law with Reynolds, Allen and Cook, a Houston-based law firm with offices in Washington. Bates will begin his new job on Dec. 1. Gay Pirozzi, formerly press secretary for U.S. Treasurer Katherine Ortega, has left to work for a private firm in London.