President Reagan and the Democratic-led 100th Congress concluded a year of bitter confrontation with grudging accommodations that left many major issues to be resolved -- or ducked again -- in the even more contentious atmosphere of an election year.

While Democrats forced Reagan into retreat on taxes and arms control, the president was able to blunt the Democrats' drive for a high-profile legislative comeback.

Together they reached a budget compromise that gives both sides an easy slide through the November elections, leaving it to the next administration and next Congress to consider big tax increases and cutbacks in popular benefit programs.

In the Iran-contra hearings, in debates over Supreme Court nominations and in the war-powers controversy over the Persian Gulf, struggles extended beyond routine fiscal and foreign policy disputes to more fundamental constitutional issues, from individual rights to the balance of powers within government. Reagan was denied his dream for an ideological turnover on the court, and Congress is counting on the embarrassments of the Iran-contra affair to curb adventurism in the executive branch. But the war-powers issue remains in dispute, and none of the basic constitutional issues was resolved.

Moreover, the session windup -- messy even by congressional standards -- prompted many lawmakers to question whether Congress itself, especially its convoluted budget process, may need a fundamental overhaul.

Reagan began the year on the defensive for the first time, wounded by the Iran-contra affair and stripped of his power base on Capitol Hill by the Democrats' recapture of the Senate in the 1986 elections. But he was able to cut his losses with the help of Senate Republican filibusters and threats of vetoes that the Democrats could not hope to override.

So, like two muscle-bound wrestlers, Reagan and the Democrats held each other in a clumsy hammerlock to the end, struggling into Christmas week -- nearly three months beyond Congress' adjournment target -- over details of policies that will be fought over again as soon as Congress returns Jan. 25.

These leftovers, including the politically explosive issue of aid to the Nicaraguan contras, will be added to the long agenda for the short campaign-year session, crowded further by Senate consideration of the new U.S.-Soviet arms treaty.

Some lawmakers saw a glimmer of light, however, citing the anticipated bipartisan support for the treaty as a sign that the second session of the 100th Congress could start in a more cooperative spirit than the first, which began with a veto confrontation over pork-barrel legislation.

Democrats also are counting on Reagan's desire to leave office on a positive note, reinforced by what they see as mounting influence of nonconfrontational moderates within the administration, to help forge agreements that were beyond reach this year.

As expected, Democratic leaders hailed the session as proof that "we can govern," as House Democratic Whip Tony Coelho (D-Calif.) put it, while Republicans dismissed the year as "pretty near a bust," in the words of House Minority Leader Robert H. Michel (R-Ill.).

But, even in their claims of success for the session, Democratic leaders voiced frustration at the difficulties of governance when the two ends of Pennsylvania Avenue are controlled by different parties.

"I never want to see divided government again in my lifetime," said Senate Majority Leader Robert C. Byrd (D-W.Va.), whose chamber was especially snarled by partisan obstructionism.

On balance, the Democrats fared better than Reagan, starting with congressional override of his vetoes of clean-water and highway bills and ending with approval of omnibus spending and deficit-reduction plans that reflected more of their priorities than his.

They forced the president to accept tax increases, military spending cutbacks and arms control constraints that he once rejected. Swallowing earlier objections, he signed legislation extending authority for independent counsels to prosecute wrongdoing within government and agreed to a compromise extension of housing programs. And the Democrats beat the president at Supreme Court politics, holding out until Reagan submitted a nominee they regard as acceptable: Anthony M. Kennedy, a federal appeals court judge from California.

But many of their biggest initiatives, such as toughening trade laws and expanding social welfare programs, including catastrophic-illness insurance for the elderly and welfare changes, remained stalled and face new hurdles in the election year. Some are threatened with vetoes that could be difficult to override.

The Democrats also had to abandon their push for a total cutoff of aid to the Nicaraguan contras and for reinstatement of the "Fairness Doctrine" for broadcasters as the price for passage of omnibus spending and tax bills that were holding up adjournment.

Earlier, they backed off from a showdown with the administration over its tanker-escort operation in the Persian Gulf and its refusal to invoke the 1973 War Powers Resolution, under which congressional approval would be necessary for continuation of the operation for more than 90 days. A suit by House Democrats to force Reagan to invoke the law was dismissed by a federal judge as a political issue stemming from Congress' lack of consensus.

Senate Democrats also had to shelve their push for campaign-finance reform for congressional elections, although Byrd vows to bring it up again next year.

In most cases, it took the intervention of outside forces -- such as the Oct. 19 stock market collapse and Reagan's meeting here this month with Soviet leader Mikhail Gorbachev -- to force a break in budget and arms control stalemates that had lasted for months.

In the wake of the Wall Street debacle, Reagan bowed to Democratic demands for budget negotiations including consideration of tax increases that he had earlier refused even to discuss. This led to agreement on a two-year, $76 billion package of deficit reductions, including $23 billion in new levies that stopped short of raising individual income-tax rates.

As the Reagan-Gorbachev summit approached, the administration flinched at the spectacle of a domestic row over arms control as the Soviet leader was arriving in town. So it agreed to a one-year limit on testing of space-based missile defenses and deployment of offensive strategic weapons, giving the Democrats a qualified victory in their campaign to force administration compliance with a narrow interpretation of the 1973 Antiballistic Missile (ABM) Treaty and the unratified 1979 SALT II pact.

The deadlock over the Supreme Court was also broken when time and patience ran out. After the Senate rejected Judge Robert H. Bork and Judge Douglas H. Ginsburg withdrew after disclosures that he smoked marijuana as a student and law professor, the Democrats issued a "third-strike-and-you're-out" warning that was heeded by the administration.

Judge Kennedy, who appeared less doctrinaire in his conservatism than Bork, sailed through Judiciary Committee hearings this month and seems on track for committee approval and confirmation by the Senate early next year.

But the record on other issues was mixed at best.

Congress approved legislation to rescue the faltering savings and loan industry and farm-credit system and approved a two-year $1 billion program to help feed and shelter the homeless, a top Democratic initiative that Reagan signed despite veto threats.

It also reauthorized elementary- and secondary-education programs, extended the authority for independent counsels to prosecute high-level wrongdoing in government and approved a ban on smoking on airline flights of two hours or less.

Legislation to expand Medicare to cover costs of catastrophic illnesses, initiated by the administration and expanded by the Democrats to include outpatient drug costs, was approved by both houses and awaits final action next year.

Less certain are the prospects for more controversial trade-law legislation that seeks to strengthen U.S. competitiveness, retaliate for unfair practices and protect businesses and workers from economic hardship caused by imports. It was passed in different forms by both houses and faces a veto threat.

The $5 billion, five-year plan for overhauling the nation's welfare system, including a large-scale education and training effort to get families off welfare and into jobs, passed the House but faces more difficult hurdles in the Senate.

An extension and tightening of the Clean Air Act, which Congress has been haggling over for six years, remained stymied, as did legislation to strengthen pesticide-control laws.

Most of labor's agenda, including a proposal to increase the minimum wage from $3.35 to $4.65 an hour, the first increase since 1981, has been put off until next year. One labor-backed proposal, which would require advance notification of plant closings, was included in the trade bill. A ban on polygraph tests for preemployment screening, notification rules for workers in high-risk jobs, mandatory parental leave and minimum health insurance requirements for all workers await action next year.