For the first time since the start of the Reagan administration, defense and foreign policy controversies are threatening to push domestic issues off center stage on Capitol Hill as Congress returns today from its 10-day Easter recess.

One reason for the shift in emphasis is the mounting congressional concern over the cost of Reagan's defense buildup, its effect on budget deficits and the squeeze it puts on social welfare programs at a time of continued high unemployment.

Another is congressional unease over the administration's arms control policies and the country's increasing involvement in warfare in Central America.

A third is that Congress took the edge off two of its biggest domestic issues with passage last month of the Social Security rescue plan and a $4.6 billion jobs program, while Reagan took steam out of the raging Environmental Protection Agency controversy with the nomination of William D. Ruckelshaus, a popular figure with many lawmakers, to head the troubled agency.

Domestic issues ranging from decontrol of natural gas prices to social welfare spending, immigration policy and repeal of tax withholding on interest and dividends remain on the congressional agenda, with the withholding issue scheduled for a Senate vote April 15.

But, with the administration bearing down on national security issues and Congress sharply divided in response, domestic issues are not likely to be the almost total preoccupation that they were in the first two years of the Reagan administration.

In the Senate, the nomination of Kenneth L. Adelman to head the Arms Control and Disarmament Agency faces continued opposition, along with a possible filibuster; Senate aides said yesterday that confirmation of Adelman remains in doubt. No date has been set for a Senate vote.

In the House a vote is expected shortly on a nuclear freeze resolution that calls for a fundamental change in the administration's policy on nuclear weapons strategy, and the resolution is expected to pass in some form, possibly next week.

And in both houses Reagan's proposal for an additional $60 million in military aid to El Salvador faces continued challenges stemming from mounting worry over widening American involvement in Central American hostilities.

Before the recess, the Senate Foreign Relations Committee voted to cut the request in half, and a Senate Appropriations subcommittee approved the full amount but attached conditions. Now the request goes before the House Appropriations subcommittee on foreign operations.

It is on defense spending, however, that Reagan probably faces his most serious and immediate trouble.

Before the recess, the Republican-controlled Senate Budget Committee was on the verge of approving a proposal to cut in half the president's proposal to increase military spending by 10 percent after inflation. But Reagan prevailed on Committee Chairman Pete V. Domenici (R-N.M.) to hold off with the lure of possible compromise, and Domenici rescheduled action for this week.

In the meantime, the Democratic House approved a budget for next year that includes an even smaller after-inflation increase in defense spending: 4 percent according to House Democrats and 2.3 percent under Congressional Budget Office figures embraced by the administration.

Senate Budget Committee Republicans, joined by Majority Leader Howard H. Baker Jr. (R-Tenn.), are scheduled to meet with Reagan today or Wednesday in an attempt to work out a compromise, a middle course between the House and administration positions.

But Reagan used the congressional recess to beat the drums for his 10 percent increase, and Senate sources were unsure whether the White House would offer enough to satisfy those who want less.

"The problem for the administration is that, if they can't get a Republican agreement, then it will be a bipartisan budget and the numbers on taxes and social spending as well as defense will be much worse for the administration," a Senate leadership aide said.