Democrats picked up 26 seats in the House of Representatives, more than enough to take back effective control from President Reagan, and in many districts did not just win, but won big.

Voters turned out of office 26 Republican incumbents, including 14 of the 52 "Reagan robot" freshmen elected on the president's coattails in 1980.

They were replaced by mostly moderate to liberal Democrats who will put pressure on Reagan to alter his legislative course.

Only three Democratic incumbents were defeated, two of them in races against Republican incumbents in contests created by redistricting.

The surviving Democrats, however, included all the Boll Weevils who helped Reagan shape and pass his economic program in the last Congress; these conservative southerners suffered no losses. On the other hand, six of the roughly 30 northern Republican Gypsy Moths who banded together against some Reagan spending cuts were beaten.

Democratic gains might have been even larger had it not been for the Republicans' financial advantage in some marginal districts, and in many races that both parties had thought would be close, the Democrats won handily. They did extremely well in areas of high unemployment, with indications that there was a strong party-line vote in some states that was intended as a message to the White House. The Republicans suffered sharp losses in Pennsylvania, for example, and saw incumbents go down in such other industrial states as Ohio, Michigan, Illinois and Indiana.

ABC News said that of 101 Republicans running in districts with unemployment greater than 10 percent, only 39 were elected and 62 were defeated.

On a regional breakdown, the Republicans suffered a net loss of 12 seats in the East, 12 in the Midwest, two in the South and none in the West.

Republican campaign officials said their candidates appeared to have been hurt also by the fairness issue, an impression among voters that the Reagan administration's programs had hurt the poor and helped the rich, and that party identification was more important in this congressional election than some in the past.

"Party label was much more important this year," said Nancy Sinnott, executive director of the National Republican Congressional Committee. "Democrats turned out. Republicans turned out. But more Democrats turned out."

The Republican losses in the House were the greatest since 1922 for a party two years into its first term in the White House. In 1922, the Republicans lost 75 seats.

By more contemporary standards, the Republican losses were more than double the average since World War II for a party in its first midterm.

Although Republican spokesmen attempted to interpret their losses in the best possible light, they could not hide their disappointment.

"I don't think we did extraordinarily well," said Rep. Guy Vander Jagt of Michigan, chairman of the National Republican Congressional Committee. "I'm not very happy."

But Vander Jagt said the loss "might have been a message to alter the course, not change the course."

His counterpart at the Democratic Congressional Campaign Committee, Rep. Tony Coelho of California, scoffed at the GOP's attempt to cast the results as something less than a defeat.

"One cannot win by losing," he said, "and it is clear that the Republican Party has lost this election. The Republicans lost because people know that the GOP economic plan is not fair and has failed miserably to produce the promised results."

House Speaker Thomas P. (Tip) O'Neill Jr. said yesterday that Reagan's conservative coalition in the House had been cracked in the final weeks of this fall's legislative session and that the Democrats would have the upper hand when Congress reconvenes.

"We extend the hand of cooperation," O'Neill said. "I'm sure we are going to be able to work with the [president]."

O'Neill also predicted that many of the conservative Democrats who helped the White House pass its tax and spending programs in the last Congress would return to the Democratic fold on key votes next year after reading Tuesday's returns.

Few of the roughly 30 Boll Weevils running for reelection were even threatened in this year's elections.

Of 35 black candidates running, 21 were elected, three more than served in the 97th Congress. All are Democrats.

Eleven black Republicans and three black Democrats were defeated. In the most closely watched race involving a black candidate, Democratic state Rep. Robert G. Clark lost his bid to become Mississippi's first black congressman since Reconstruction.

Women added to their strength in the House, raising their total from 20 to 21, despite three retirements.

In six races that pitted incumbents from each party against the other, the Republicans lost four. In California, Democrat Matthew G. Martinez beat Republican John H. Rousselot; in South Dakota, Democrat Thomas A. Daschle beat Republican Clint Roberts; in Missouri, Democrat Ike Skelton beat Republican Wendell Bailey; and in Massachusetts, Democrat Barney Frank clobbered Republican Margaret M. Heckler.

Only in New York did the Republicans win the head-to-head contests, as Republican Guy V. Molinari defeated Democrat Leo C. Zeferetti, and Republican Benjamin A. Gilman beat Democrat Peter A. Peyser.

Republicans saved their most prominent endangered incumbent, House Minority Leader Robert H. Michel of Illinois, who squeaked out a victory over union lawyer G. Douglas Stephens in a central Illinois district smarting from unemployment and sagging farm prices.

But Michel was one of the lucky GOP incumbents, as voters picked off one after another throughout a long Tuesday night of counting that saw Republican prospects go down, jump back up and finally roll back down again.

One Republican who fell early was Delaware's Thomas B. Evans Jr., who was hurt by his association with Playboy model and Washington lobbyist Paula Parkinson. He was defeated by Democratic state Treasurer Thomas R. Carper.

One who fell late was Don H. Clausen of California, who had been targeted by supporters of the nuclear freeze movement and lost to state assemblyman Douglas H. Bosco.

The only other Democratic incumbent to lose was Ohio's Bob Shamansky, to Republican state Sen. John R. Kasich.

In past years, incumbents have been mostly invulnerable to defeat, with roughly 90 percent who sought reelection winning. But on Tuesday, Republican incumbents broke that pattern.

Most surprising was the defeat of 14 members of the Republican class of 1980, who had helped Reagan pass his program but ran for reelection by distancing themselves from the president. In recent years, freshman House members seeking reelection had won by larger percentages than their more senior colleagues, but on Tuesday they seemed like ducks in a shooting gallery.

They included New York's John LeBoutillier, one of the brashest of the class, who lost to Suffolk County legislator Robert J. Mrazek, as well as West Virginia's Mick Staton.

Four of those freshman Republicans were in districts where the black vote was considered important and where blacks had targeted the Republican for defeat. Those losers included Albert Lee Smith Jr. of Alabama, John L. Napier of South Carolina, and Eugene Johnston and Bill Hendon of North Carolina.

North Carolina was a particular disppointment for the Republicans Tuesday. Not only did they see two incumbents defeated, but they failed to pick up a seat they expected to win against Democrat Ike Andrews, who had been weakened by a drunken driving charge shortly before the election.

Four more Republican incumbents fell in Pennsylvania. Two were anticipated: James K. Coyne, a target of groups supporting a nuclear freeze, who lost a rematch to former Democratic Rep. Peter H. Kostmayer, and Eugene V. Atkinson, who jumped to the Republican party at the height of Reagan's strength in Washington and was rewarded with forced retirement. But two were surprises: Charles F. Dougherty and James L. Nelligan.

In other areas where the economy was especially weak, Republicans failed to survive, even when expected to do so. In Ohio, freshman Rep. Ed Weber lost badly to former Carter White House aide Marcy Kaptur. In Connecticut, liberal Republican Lawrence J. DeNardis was an apparent loser to legal aid lawyer Bruce A. Morrison. In New Jersey, Republican Harold C. Hollenbeck lost to Robert G. Torricelli, a former aide to former vice president Walter F. Mondale. In Indiana, Joel Deckard was defeated by Bloomington Mayor Francis X. McCloskey.

A sign of just how much incumbents were affected by their association with the policies of the Reagan administration came in two open districts in the Northeast. At a time when GOP incumbents were losing in nearby districts, Republicans Thomas J. Ridge held a seat in Erie, Pa., and state Sen. Nancy Johnson captured a Democratic seat around New Britain, Conn.

Farm prices and the rural economy appeared to have cost Republicans the seat held by Paul Findley in Illinois, the apparent loser to lawyer Richard J. Durbin. In Minnesota, Republicans Tom Hagedorn and Arlen I. Erdahl were defeated in rural districts.

Where once the Republicans expected redistricting to help them, they said yesterday it apparently had hurt. One GOP official estimated that half the seats they lost came in districts that had been weakened by redistricting. One prominent example was in Michigan, where freshman Republican Jim Dunn lost a rematch to Democrat Bob Carr in a district that was redrawn to include Pontiac, where unemployment has been running at 28 percent. Another loss attributed to redistricting was Rousselot in California.

In the 58 open districts where there was no incumbent running, the Democrats picked up seats in Kansas, Texas, Arizona, California, Florida, Illinois and elsewhere.