President Reagan put aside coalition politics today to campaign for a Republican Senate candidate who opposed his tax bill against a Democrat who favored it.

At the same time, Reagan prepared to veto what White House deputy press secretary Larry Speakes called a "budget-busting" $14.1 billion supplemental appropriations bill.

Another administration official predicted that the veto would be sustained along partisan lines and would help restore the Republican unity shattered last week when the White House entered into a coalition with the House Democratic leadership to push through a $98.3 billion tax increase.

Reagan came down by helicopter from his mountaintop ranch near Santa Barbara today to make the featured speech at a western-garbed, $1,000-a-person fund-raiser for San Diego Mayor Pete Wilson, the Republican nominee for the Senate against Democratic Gov. Edmund G. (Jerry) Brown Jr.

Speaking from a Twentieth-Century Fox sound stage to a western-garbed audience, Reagan gave a rambling and uncharacteristically listless speech devoted more to defense of his administration's record on the economy and national defense than to the Brown-Wilson race.

When Reagan did talk about the California campaign, it was more in terms of his distaste for Brown, who succeded him as governor in 1975, than his admiration for Wilson.

"You send Pete Wilson back to Washington and, if you can't send him, don't send anybody," Reagan said.

The president depicted Brown as a big spender in the mold of Sen. Edward M. Kennedy (D-Mass.), who will campaign for Brown, and said of the California governor: "Pay attention to what he does and not what he says."

The affair was supposed to raise $1.5 million for Wilson's campaign coffers. However, sources close to the campaign said today that Wilson's opposition to the tax bill had irked key longtime Reagan backers, who withdrew their support from ticket sales to the fund-raising event.

As a result, the sources said, the event would take in $500,000 less than had been anticipated.

Wilson probably has suffered more from his opposition to the tax bill than have any of the 89 GOP House members who voted against it. Wealthy Reagan backers in California have long questioned the San Diego mayor's basic loyalty to Reagan because they remember how strenuously he campaigned for President Ford against Reagan in the 1976 New Hampshire primary.

Nonetheless, the Reagan watchword today was unity. White House spokesmen are making clear that there will be forgiveness and campaign support for House conservatives -- or any other Republicans -- who opposed the president on the tax bill.

"We have a big tent and they're all welcome back under it," Speakes said.

Apart from any personal feelings about Wilson, the White House and Republican strategists recognize that keeping GOP control of the Senate is a top political priority for Reagan. They are concerned that Wilson, who now leads Brown by only six percentage points in private GOP polls, is losing his advantage despite Brown's high negatives with many California voters.

As a result, Reagan and White House political operatives have promised to do all they can to defeat Brown and elect the San Diego mayor to the Senate. At the request of White House political operatives, Stu Spencer, Reagan's chief strategist in the 1980 campaign, has been called in to assist Wilson's campaign.

In discussing the supplemental appropriations bill today, Speakes stopped short of forecasting a Reagan veto this week. But his critical description of the bill confirmed what other White House advisers have been saying, which is that the president is certain to reject the measure. This would quickly dispel any notion that he intends to become a coalition president on the model of Dwight D. Eisenhower, as some suggested in the wake of last week's tax vote.

House Majority Leader James C. Wright Jr. (D-Tex.) has said it would be "very ungracious" for Reagan to veto the measure so soon after the Democrats helped him push through his tax bill.

Reagan will follow up his campaign appearance today with a speech here Tuesday for Republican gubernatorial candidate George Deukmejian. On Wednesday morning the president will return to his ranch, to vacation until after Labor Day.

Speakes announced today that Reagan would resume his five-minute Saturday radio broadcasts, which he concluded last June with a speech from Versailles. The original 10-week series of broadcasts was judged a success by White House advisers, who said it gave the president a chance to present his views in a familiar format every week, and not coincidentally, provide a story for Sunday newspapers.

Reagan, who got his start as a radio sportscaster and delivered a regular political radio commentary after he left the California governorship, liked doing the speeches, which he usually wrote himself. One aide said that Reagan himself had pressed for resumption of the radio talks.

The president is scheduled to visit the Los Angeles offices of Dr. Howard House today for what the White House described as a "routine ear examination."