Heading into a pivotal election year and the final months of their long power struggle with a Republican president, the Democrats who control Congress yesterday began what may be the difficult task of drawing the curtain on the Reagan presidency.

In their responses to Reagan's final State of the Union address last night, House Speaker Jim Wright (D-Tex.) and Senate Majority Leader Robert C. Byrd (D-W.Va.) sought to paint a sharp contrast between the priorities of the Democratic-led Congress and the final-year policies of an administration they believe has lost its ability to hold sway on Capitol Hill.

"We've come to the end of an era," said Byrd. "The 'feel-good' slogans have gone flat with time. We've learned that bravado is not leadership -- that ideology is no substitute for common sense."

Referring to the nation's huge trade and budget deficits, Byrd said that "the dark side of the Reagan years has only begun to loom."

Laying out the Democratic agenda yesterday, Wright said his party would pursue policies "that are important to the American people" by pushing for peace in Central America and a number of pieces of legislation, including bills to reverse the nation's trade imbalance, improve its schools, put welfare recipients back to work and protect the elderly from catastrophic illnesses.

Evoking memories of his upbringing during the Depression and of how President Franklin D. Roosevelt "lifted the spirit of America," Byrd, speaking on the Senate floor, crafted an unflattering comparison with the "profound experiment" of the Reagan years, which he said was "born of ideology and a technicolor view of America and our people."

Byrd also pointedly referred to the Iran-contra scandal and the "cases of cronyism and abuses of power for personal gain" that have dogged the administration. The time has come, he said, for a government interested in "helping those in need, not those with greed."

Nonetheless, even some Democrats admitted that Reagan, though weakened by the ending of his presidency and the aftertaste of the Iran-contra affair, may still have considerable clout in Congress, particularly on foreign affairs issues that may dominate this year.

"On the major issues, the economy and arms control, his power has not diminished," observed Sen. Edward M. Kennedy (D-Mass.), noting that Reagan would benefit from the expected Senate approval of his arms control treaty with the Soviets to eliminate intermediate-range nuclear missiles in Europe.

Sen. Alan Cranston (D-Calif.), the assistant majority leader, said, "There will be perhaps a greater balance than there was in his first term, but he will have plenty of impact."

Other factors that could contribute to a resilient final year for Reagan are a shortened congressional domestic agenda that consists largely of wrapping up unfinished legislation from last year, and the power of congressional Republicans to uphold presidential vetoes of controversial measures.

"The president is still the president and I still think he has a whale of a lot of support out there in the country," said House Republican Leader Robert H. Michel (R-Ill.). "This isn't the time to sing any swan song."

"Clearly we didn't hear from a lame duck tonight," agreed Rep. Dick Cheney (R-Wyo.), who said the president's plan to ask Congress to retroactively remove objectionable features of last year's massive catchall spending bill "is going to embarrass the hell out of some people."

Reagan's call for budget reform -- a point driven home when he hefted two spending and tax bills passed on Congress' final day -- found a receptive audience. Though most lawmakers said his renewed calls for a line-item veto and a constitutional amendment requiring a balanced budget are dead letters, many appreciated his demand they not send him any more omnibus appropriations bills.

"A picture is worth a thousand words and tonight America saw a continuing resolution," said Sen. John W. Warner (R-Va.), using Congress' term for catchall spending bills. "We got caught red-handed."

Even before Reagan's speech, Democratic leaders had pledged they would pass all 13 regular appropriations bills this year in place of an omnibus measure.

Democrats yesterday made a concerted effort to highlight the weaknesses of the president and their ability to shape the national debate.

"The agenda will be set by Congress," said House Majority Whip Tony Coelho (D-Calif.). "In his eighth year, {Reagan} doesn't have the ability to set the agenda. He's basically lost clout in his own party, never mind our party. He's looking to his historical record. We are looking ahead."

Sen. George J. Mitchell (D-Maine) added, "This president has pretty much run out of steam . . . . The president's greatest influence will be a negative one through the use and the threatened use of the veto."

The tone for the final-year clash of wills will be set next week, when the House takes up Reagan's request for additional aid to the Nicaraguan rebels, a showdown that Democrats began casting today as one that would highlight their differing priorities.

"It's unfortunate that the president is beginning his last year on such a confrontational tone by devoting so much to contra funding," said Mitchell. "Cooperation and a productive last year would be good for this president, the presidency and the country."

"The president will say one of his top priorities is giving money to the contras," said Coelho. "It's not taking care of farmers, kids, AIDS or the homeless. He will give up everything to help the contras."

Democrats severely criticized the president's expected request for some military aid for the Nicaraguan rebels that would be escrowed, its release tied to continued progress toward peace in Central America.

"The president has pitted himself against the five Central American presidents," said Rep. David E. Bonior (D-Mich.), a member of the Democratic leadership team who heads his party's task force on the Nicaraguan issue. "He has to reconcile whether he is for a peace treaty or against it."

The Democrats' attempt to highlight their differences with Reagan and portray themselves as in control was field-tested several times over the past few days.

Yesterday, for example, Wright three times reviewed in detail not only his party's agenda for the coming year but its accomplishments last year.

Congress, said Wright, had protected the environment and improved transportation by passing, over Reagan's vetoes, clean water and highway legislation. In promising to complete action on trade legislation that would require retaliation against nations that do not engage in fair trade, he belittled the president's lack of concern for the trade deficit. And, reminding voters that the president had sought to cut education spending 28 percent last year, Wright pledged that Congress this year will pass legislation to improve educational programs.