After six months of warm-ups, Congress returned from its Fourth of July recess yesterday to face a calendar jammed with tough choices and little time for making them.

In the four weeks before its month-long August break, Congress is to consider final compromises on deficit reduction, defense spending and aid to antigovernment rebels in Nicaragua, as well as sanctions against South Africa, farm program overhaul, relaxation of gun controls, immigration law revision, several appropriations bills for next year and a wrap-up spending measure for this year.

"We'll be enormously busy," said House Majority Whip Thomas S. Foley (D-Wash.), speaking of Congress' traditional midsummer rush to clear its decks of major legislation before moving onto consideration of appropriations bills after the August recess.

Overshadowing everything else is the budget, including $56 billion in spending cuts aimed at drastic reductions in federal budget deficits hovering around $200 billion a year. Without a budget compromise, many other important pieces of legislation could be complicated, delayed or even jeopardized.

To help break a House-Senate deadlock on the budget, President Reagan has scheduled a series of meetings this week with principal congressional figures in the budget fight, including one session with leaders of both parties today and another with budget negotiators Wednesday.

Senate Majority Leader Robert J. Dole (R-Kan.) welcomed Reagan's intervention, expressing hope that the White House meetings would serve as an "icebreaker." Dole also said he is interested in trying to work with House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) to help budget conferees reach agreement.

Many lawmakers said yesterday that they sensed increased pressure among constituents during the holiday recess for action to bring deficits under control.

"The deficit is starting to drive people up the wall for the first time," Rep. William B. Richardson (D-N.M.) said. "People are saying, 'Cut the deficit and stop playing politics.' "

Lawmakers said they sense less pressure for action on Reagan's plan for tax-code overhaul, especially as constituents become increasingly aware of how the proposed changes would affect them personally.

"A lot of opposition is developing against the [Reagan] proposal in its present form," said Rep. John J. Duncan (R-Tenn.), ranking minority member of the House Ways and Means Committee. "I don't think constituents are in a big hurry for a tax bill."

Dole said he agrees that pressure for the tax bill is receding as concern increases about deficits. While people think "we've gone bonkers here" in delaying action on the budget, Dole said, "you almost have to ask them about the tax bill" to elicit any reaction to it.

The Senate began its midsummer push by considering sanctions against South Africa, approved in stronger form by the House. But it ran into delaying tactics from conservative Sens. Jesse Helms (R-N.C.) and Steve Symms (R-Idaho).

A cloture petition to force consideration of the measure was filed, and the Senate is to suspend debate on the bill today in order to act on legislation that would relax federal controls over sale and transportation of firearms.

The House is expected to spend the week on next year's foreign aid authorization bill, passed in different form by the Senate. Dozens of amendments are being proposed on controversial issues ranging from aid to Nicaraguan rebels to repeal of the so-called Clark amendment, which prohibits U.S. assistance to Angolan insurgents.

Other amendments would cut off further military aid to Lebanon until the seven U.S. hostages held there are released and deny U.S. funds to organizations that promote or perform abortions as a method of family planning.

Later, the House may take up the 1985 intelligence authorization bill, which would force another examination of aid to Nicaraguan insurgents. The House also expects to consider at least five appropriations bills for next year, covering most of the government's domestic agencies, and the Senate may act on some of them.

The Senate calendar for the month includes immigration-law revision, which died in a House-Senate stalemate last year, and legislation authorizing the president to veto individual items in appropriations bills that must now be signed or vetoed as a whole.

Dole said he also intends to introduce legislation to overhaul and cut back farm programs.

The Senate Agriculture Committee has given itself until Monday to draft a bill, but the measure is so controversial that senators are skeptical about meeting the deadline. Similar legislation may come up in the House, although leadership officials said action is unlikely before the August recess.

Still to be resolved by House-Senate conference committees are several major bills in addition to the budget.

They include the fiscal 1986 defense authorization, for which compromises must be negotiated on the MX nuclear missile, the president's Strategic Defense Initiative for a space-based defense against missile attack, the death penalty for members of the armed forces convicted of peacetime spying and cost controls for Defense Department weapons purchases.

At issue in the conference on the supplemental money bill for this year are funds for Nicaraguan rebels and cost-sharing requirements for water projects. Both could cause protracted haggling, according to sources in each chamber. In addition, the Senate approved nonmilitary aid to Jordan, while the House took no action on the issue.

One point of dispute is whether the Central Intelligence Agency should handle the Nicaraguan funds, a move the Senate supports and the House opposes.

Also, the Senate wants to force local governments and commercial users to pay part of the cost of water-project development, while the House did not include such language in its version