Even before Ronald Reagan was elected president, deficit-conscious conservatives of the old school were engaged in a struggle for his heart and mind with the tax-cutting apostles of the supply-side church.

The traditional conservatives were more pragmatic and less bullish on the economy than the supply-siders. These pragmatists, epitomized in the White House by chief of staff James A. Baker III and on Capitol Hill by such Republican Senate leaders as Finance Committee Chairman Robert J. Dole (Kan.), lost some battles but won a notable victory last August when they persuaded Reagan to mount a successful campaign on behalf of a $99 billion tax increase and reform measure.

The supply-siders, of whom Rep. Jack Kemp (N.Y.) has been the recognized congressional leader, gradually have lost most of their early advocates within the administration.

However, they often have been able to count on Reagan's unswerving devotion to income tax reduction.

While the president has been, to put it mildly, laid back in resolving internal conflicts of his staff and Cabinet, he has usually been a "majority of one" on the issues that mean most to him: military spending and cutting income taxes.

What has changed, as the administration nears midterm, is less the nature of the struggle than the attitude of the participants.

Though the November elections made the pragmatic case for midcourse correction, they appear to have propelled the president in the opposite direction.

In fact, the results may have hardened the president and softened the pragmatists. While Reagan seems more determined than ever to cut taxes and keep the defense budget growing, those who fought to change Reagan's mind on the tax issue last summer show no inclination to wage an in-house fight for even modest defense cuts or tax reduction deferrals.

The pragmatists instead rely on the solace of history. They know that Reagan, when reelected governor of California in 1970 while Democrats were capturing the state Assembly, modified his program and produced important compromise legislation on welfare reform, taxes and education.

But that was 12 years ago, and Reagan soon will be 72 years old. His vaunted sense of timing may be telling him that time is running out. If so, he could become even more intransigent, because Reagan is a competitor who won't willingly abandon cherished programs just because they have become politically unpopular.

"We came to Washington to drain the swamp," Reagan is fond of saying these days. If he is in a mood to compromise, he has yet to convey this to his staff.

Remember last June when an exhausted president of the United States, in a historic meeting with Pope John Paul II at the Vatican, briefly nodded and almost turned a public relations triumph into a disaster? Several White House officials remember only too well and are worried that Reagan again is being overscheduled in advance of a foreign trip.

Today, the president is scheduled to arise early at his Santa Barbara ranch and to make a speech on urban issues in Los Angeles before flying back to Washington early tonight. He departs Tuesday on a 9 1/2-hour flight to Brazil, with briefings scheduled on Air Force One. The White House tried unsucessfully to change today's speech to last night, but the sponsoring National League of Cities wouldn't budge. Reagan then decided to give the speech as scheduled.

One administration official, trying to make the best of it, observes that the South American trip, unlike the European odyssey, is short on evening ceremonial functions.

This time, the official says, Reagan will "get to bed at a decent hour."

There's no shakeup in sight at 1600 Pennsylvania Avenue, but the White House would like to see one at the Senate Republican campaign committee, where Richard G. Lugar (Ind.) is challenging Bob Packwood (Ore.) for the chairmanship. Reagan remembers what he regards as Packwood's personal slurs about his abilities, and he and his aides are rooting hard for Lugar. But it's also widely recognized at the White House that any attempt to go beyond cheerleading could wind up being counter-productive.

Despite pressure from the Labor Department and the insurance industry, the administration has dodged a political bullet in deciding not to join Arizona in a key case before the Supreme Court.

When a state-deferred compensation plan provided smaller annuities for women than men on grounds that women live longer, women employes sued and won on grounds of sex discrimination. Siding with them as a friend of the court was the Equal Employment Opportunities Commission. After high-level consulations with the White House last week, the Justice Department decided not to intervene on the side of Arizona before the high court.

The decision produced a collective sigh of relief at the White House. As one aide put it:

"Thankfully, we passed up the opportunity to widen the gender gap with a single stroke."