When the Democrats lost control of the Senate six years ago, ending a quarter-century of hegemony over both houses of Congress, Senate Democratic Leader Robert C. Byrd (D-W.Va.) gamely suggested it might turn out to be a "healthy shock" for the party.

Shock it was, healthy or not.

Even Senate Democrats concede that they are only now emerging from a wrenching trauma, still struggling to learn how to function effectively as a minority within the institution and still groping for a new image to present to the country.

As they head toward elections this fall that could return them to majority status, some concede they never really got the hang of being a minority.

"We haven't articulated acceptable alternatives in most cases . . . . We've only grudgingly conceded some of the reasons we found ourselves in a minority," said Sen. Dale Bumpers (D-Ark.).

"We're doing better, but we still have not reached the degree of cohesion that the Republicans have," said Sen. George J. Mitchell (D-Maine).

President Reagan "has been bold in his ideas and his criticism of Democrats, and our reaction has been to run for cover and say he's a Teflon president . . . . We've been too timid, too afraid of the president," Sen. J. Bennett Johnston (D-La.) declared.

"It took us four years to accept the fact that we were in the minority; some of our older members haven't accepted it yet," said Sen. Jim Sasser (D-Tenn.).

In a divided Congress, the Senate Democrats are the lowest of the low, overshadowed by the Republican majority in the Senate and the Democratic majority in the House, without the links to the White House that the House Republican minority can claim.

With hopes of returning to power this fall, Senate Democrats cannot even share in the hopelessly outnumbered House Republicans' luxury of bomb-throwing and other mischief-making; they must look like a government-in-waiting.

Even in the role of loyal opposition, they come in second not only to House Democrats but often also to Senate Republicans, whose rebellious departures from fealty to the White House have gotten more limelight than the Democrats' more predictable moves in the same direction.

Adding insult to injury, House Democrats cut their deals with Senate Republicans; the White House deals with Senate Democrats mainly when it needs to plug gaps created by GOP defections.

With their world turned upside down by Reagan and the coattail effects of his landslide victory in 1980, Senate Democrats have been gun-shy of the president, often supplying enough Democratic votes to assure him legislative victories in the Senate.

They have rarely presented party-backed alternatives to Reagan programs, claiming that, in holding only 47 of the 100 Senate seats, they lack the votes to prevail. But, even when enough moderate-to-liberal Republicans have peeled away from Reagan to give the Democrats a majority, they often split apart themselves. Consensus has been difficult to achieve, agreement on specific alternatives virtually impossible.

With death and defeat eroding both their right and left wings, the Senate Democrats are arguably now more homogeneous than the Republicans, who run the ideological gamut from Jesse Helms (N.C.) on the right to Lowell P. Weicker Jr. (Conn.) on the left. Sens. Howell Heflin (D-Ala.) and Edward M. Kennedy (D-Mass.), who are at or near the current Senate Democratic extremes, both vote to sustain contested civil rights laws; both also voted for the budget-slashing Gramm-Rudman-Hollings bill.

"We've gotten rid of some of the ideological baggage of the left and right . . . . The liberals are not as liberal; the conservatives are not as conservative," Johnston said. "The Democratic Party as a whole has become more centrist," agreed Byrd.

But Senate Republicans, with a discipline partly rising out of their long years in the minority, are more likely to stick together when it counts, many Democrats concede. "Republicans were in the minority so long they learned to stick together," Bumpers said. "The Democrats always had such a big majority they could afford six or seven defections. Now, occasionally we can conspire to stop something that's really bad, but it's an exception."

Many Democrats also concede that the Republicans, in choosing Sen. Robert J. Dole (Kan.) as their leader, present the image of an activist, creative, take-charge party. By sticking with the lower-profile, looser-reined leadership of Byrd, the Democrats bought more time to sort themselves out. And that, some complain, is the image they present: "a collective identity crisis," in the words of an aide to one of Byrd's leading critics.

When they lost the Senate, the Democrats went into a deep funk that many of them say lasted until 1984, when Reagan's reelection gave what Sasser called "an air of finality" to the Democrats' plight.

Democrats had lost some of their strongest figures in the 1980 elections, and only their two most senior members -- John C. Stennis (Miss.) and Russell B. Long (La.) -- had served in the Senate when Democrats were in the minority, back in the mid-1950s.

"They thought it was their God-given right to run the Senate and that somehow 1980 was an aberration," said Norman Ornstein, congressional scholar at the American Enterprise Institute.

Gradually, however, especially this year, they have begun to pull together and to seize opportunities as they arise. Along with frustration over earlier failures, they appear energized by the rare opportunities in this fall's elections, when Republicans will be defending nearly twice as many seats as Democrats. "The idea of going into the '90s in the minority focused their attention tremendously," Ornstein said.

In the past year, Democrats forced action on farm credit relief, left a big mark on tax-overhaul legislation, helped reshape the Saudi arms sale, drummed up pressure for trade action, defied the administration in the still-unresolved fight over confirmation of conservative judicial nominee Daniel A. Manion and, perhaps most importantly, profoundly influenced the fiscal 1987 budget that came out of the Senate.

Unable to pass a budget of their own because of splits in their ranks, Republicans had to pay a high price for Democratic votes in the form of a top-to-bottom repudiation of Reagan's budget priorities.

Much of the Democrats' success came not through consensus, but through solo efforts of Democratic senators such as Bill Bradley (N.J.) on tax reform, Lawton Chiles (Fla.) on the budget and Sam Nunn (Ga.) on defense policy. When agreement among Democrats proved impossible, these lone rangers rode out on their own. More often than not, they have been followed, eventually, by Republicans as well as Democrats.

"It was difficult to get a consensus with any kind of cutting edge . . . when the party was still trying to sort out its position," said Bradley, speaking of his push for tax reform and simplification as head of the Democratic caucus' economic task force in the early 1980s. So Bradley moved out on his own, and, when the tax-overhaul bill emerged from the Senate Finance Committee last spring, it bore a close resemblance to what Bradley had been talking about all along.

But the tax debate also revealed the schisms that remain for Senate Democrats.

A key issue as the measure came to the Senate floor involved a move by Mitchell to add a higher rate for upper-income taxpayers, which would make the measure more progressive but jeopardize the delicate political balance necessary to pass the whole bill.

Arkansas' two Democratic senators split on the issue for reasons that go to the heart of the party's struggle to reconcile old values and new needs.

"If Democrats don't stand for a progressive income tax , they don't stand for anything," said Bumpers. Arkansas colleague David Pryor conceded Bumpers' argument in theory but asked, "Does the Democratic Party want to be seen as standing in the way of tax reform . . . of once again being anti-success?"

In the vote on Mitchell's proposal, 22 Democrats agreed with Pryor and, with Republicans nearly unanimous against the change, it was defeated, 71 to 29. Not so long ago such a split would have been hard to imagine. Senate Democrats simply were not expected to pass up a chance to soak the rich to give breaks to everyone else.