Facing an election-year budget struggle of its own making, the 99th Congress reconvenes today for what is expected to be the toughest and tensest session of the Reagan era.

Congress is expected to be dominated from start to finish by the rigorous budget-balancing law that it imposed on itself last year after a frustrating and inconclusive effort to reduce the deficit. What lies ahead may be worse.

"People are looking at this session with real dread," said Rep. Les AuCoin (D-Ore.). "I think this is going to be the worst Congress of my six terms, just in terms of frustration, chaos and irritation."

Everything from national security to environmental protection is likely to be affected by the budget debate. Reagan's military buildup will be threatened and his tax-overhaul proposal could be turned into a tax increase before the year is out.

The balanced-budget straitjacket will put extraordinary new pressures on the strained relations between the White House and Congress, while the fragile institutional bonds within Congress will be stretched to the breaking point, some lawmakers predict.

"If last year was 'Morning in America,' " said Christopher Matthews, aide to House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.), referring to President Reagan's 1984 campaign slogan, "then this year is the morning after."

Politically, there is much at stake, with Republican control of the Senate most significant. The struggle for Democratic leadership posts in the House also could get caught in the budget undertow, and the fortunes of several possible presidential contenders for 1988, including Senate Majority Leader Robert J. Dole (R-Kan.), Rep. Jack Kemp (R-N.Y.) and Sen. Gary Hart (D-Colo.), could be affected.

"There's likely to be a lot of china broken around here before it's over," Dole said.

The first session of the 99th Congress, which lasted two months longer than planned and ended in fiscal stalemate shortly before Christmas, was marked by few substantive accomplishments, principally the enactment of a new farm bill and House approval of tax-overhaul legislation.

Its hallmark was passage of the Gramm-Rudman-Hollings bill to force a balanced budget by fiscal 1991. Many lawmakers regarded the action as a congressional confession of impotence in dealing with the distasteful choices required to curb deficits soaring beyond $200 billion a year.

The measure, named for Senate sponsors Phil Gramm (R-Tex.), Warren B. Rudman (R-N.H.) and Ernest F. Hollings (D-S.C.), sets fixed deficit targets for the next five years and requires automatic spending cuts if Congress fails to meet them.

It was a kind of political promissory note that enabled Congress to survive another year without the pain of heavy spending cuts or tax increases. But the respite was short, with deadlines for compliance beginning to roll even before lawmakers started straggling back to town for the new session.

Mandatory spending cuts totaling $11.7 billion for the rest of fiscal 1986 were unveiled last week and will take effect March 1 unless Congress and the president produce an alternative blueprint before then. Meanwhile, on Feb. 3 or soon thereafter, Reagan will submit a budget for fiscal 1987, setting off new deadlines that could trigger more than $50 billion in automatic cutbacks by early October.

Reagan's budget is expected to contain a substantial increase in defense spending and reductions in domestic spending previously rejected by Congress, but Congress is likely to find it unacceptable. If Congress finds itself stymied in trying to draft an alternative, there will be growing pressure for summit-like negotiations with the White House on a compromise. That, however, could be months away, for Reagan appears in no hurry to cut a deal.

When Congress adjourned last month, members were dispirited and disgruntled, and the enormity of their new task is unlikely to improve spirits. Second thoughts about Gramm-Rudman-Hollings quickly surfaced, but lawmakers say that the political price of ducking the consequences could be as great as grappling with them.

Some members are looking to the courts to save them, but a decision on a pending lawsuit against Gramm-Rudman-Hollings is not likely to provide political immunity. Even if the courts disallow the triggering device for the automatic cutbacks, Congress will be required under the law to enact the cutbacks if it has not met the deficit target.

"I can see such chaos by late summer that people will seriously start thinking about squirming out of the Gramm-Rudman-Hollings law," one high-level congressional aide said. "But voters will see that as a cop-out, and they may 'throw the bums out,' regardless of party, regardless of whether they're House or Senate members," he added.

That could lead to a reversal of the relatively recent pattern of high reelection rates for congressional incumbents. But if polls indicate a voter revolt in the offing, that also could serve as a powerful inducement to stick with Gramm-Rudman-Hollings, regardless of the other consequences.

Opportunities for political gamesmanship abound. Senate Democrats, eager to pick up the four votes necessary to regain control of their chamber, are likely to try to pressure the Republican majority to comply with the letter as well as spirit of the law. House Democratic leaders and Senate Republican counterparts may try to play Alphonse-Gaston over who makes the tough moves first, but neither can risk appearing to be the spoiler.

Some critics have charged that the budget legislation will, in effect, put Congress on "automatic pilot," but it appears instead that the measure only adds to the unpalatable choices facing members. Perhaps the most difficult choice is whether to raise taxes to help shield both defense and domestic programs from the guillotine of Gramm-Rudman-Hollings.

Already there is talk of an oil import fee and of using the tax-revision bill pending in the Senate to raise revenue, even though Reagan has demanded that any such tax bill be "revenue-neutral." The sticking point on taxes is Reagan. Congressional leaders of both parties refuse to move on raising taxes unless Reagan helps lead the march, something he thus far has refused to do.

Plagued by its political divisions and by disputes with the White House, Congress has increasingly been able to act only against deadlines that threaten massive governmental disruptions. The Gramm-Rudman-Hollings deadlines for fiscal 1987 will not truly be felt until fall. As a result, many lawmakers fear months of dissension and drift, climaxed by a nerve-jangling confrontation only a month before the congressional elections.

"What you've got is an extraordinary set of circumstances on top of a situation that is sufficient in itself to destroy the session," said a House Republican leadership aide, referring to ordinary budget pressures, the November congressional elections, intraparty struggles and Reagan's lame-duck presidency. Even the imminence of the 100th Congress, a symbolic milestone that will invite critical introspection, may send shudders through the institution, the aide added.

Despite all this, the formal proceedings of the two houses will open on a deceptively routine note. The Senate will consider legislation to sell the Conrail system (with "probably the first filibuster of the year," according to Dole), and the House will take up a banking bill.

The Senate then will move to legislation paving the way for radio and television coverage of its proceedings, a constitutional amendment to balance the budget and the long-delayed antigenocide treaty. The House has nothing scheduled beyond the banking bill, aides said.

The Senate plans lengthy hearings on the tax bill, with floor action not expected until late spring or early summer. But both houses must deal more quickly with another major leftover from last year, the budget "reconciliation" bill that would reduce spending and raise taxes by about $75 billion over three years. One option under consideration is to substitute this measure for the domestic portion of the automatic Gramm-Rudman-Hollings cutbacks required for the current fiscal year, although such a move might open Congress to charges of fudging from the start on compliance with the law.

Facing Congress as the year moves on are several major foreign policy issues, including aid to Nicaraguan rebels and arms sales to Arab countries. The administration is expected to press for perhaps $100 million in both lethal and nonlethal aid to anti-Sandinista forces in Nicaragua. Last year, Congress approved $27 million in "humanitarian" aid, but the funding authorization expires March 31.

A ban on arms sales to Jordan expires March 1, and there will be strong congressional resistance to expected administration moves to sell weapons to both Jordan and Saudi Arabia until there is more progress on Mideast peace talks with Israel.

Also expected is renewed debate over sanctions against South Africa, covert aid to antigovernment forces in Angola (a ban on it was lifted last year) and the effectiveness of U.S. assistance to anti-Soviet forces in Afghanistan. Designation of a congressional delegation to monitor elections in the Philippines is possible, along with continued pressure for democratic reforms.

The administration will press for funds to strengthen security of U.S. facilities abroad, along with antiterrorist aid for Central America. There also may be a renewal of last year's pressure for more loan foregiveness for Israel. But the budget-balancing law is likely to put a damper on new foreign aid ventures.

Although some of the steam has gone out of the last year's protectionist drive, major trade legislation will be considered, including an overhaul of trade procedures. Congress, hoping to pressure the administration to negotiate new trade pacts, has put off until August a vote to override Reagan's 1985 veto of a textile protection bill.

A new effort to resolve Senate-House differences over extension of the "Superfund" toxic-waste cleanup program, including taxes to finance it, is expected, along with moves to complete action on long-delayed water quality legislation.

Civil rights advocates will push to resolve an abortion dispute that is holding up House action on a civil rights bill that would overturn a 1984 Supreme Court decision that restricted enforcement of antidiscrimination laws.

The House will consider its version of Senate-approved immigration legislation when and if a dispute over temporary foreign workers is resolved. Election-law revision is also on the agenda, including public financing of congressional elections and restricting political action committee campaign funding.