After a dramatic parliamentary gamble, Senate Democrats lost a prime opportunity to defeat judicial nominee Daniel A. Manion yesterday when Sen. Slade Gorton (R-Wash.) switched his vote after being promised that the White House would nominate his choice for a federal judgeship in his state.

The Democrats' failure by a single vote to defeat Manion outright led to a parliamentary deadlock. By day's end the Democrats had dropped their plans for a filibuster and agreed to another up-or-down vote on Manion after Congress returns from its July 4th recess.

No longer faced with the need to find 60 votes to cut off debate, Senate Republicans said they think that they have the votes to confirm Manion, President Reagan's choice for the 7th Circuit Court of Appeals in Chicago.

A Republican leadership aide said the Democrats "tried a quick checkmate and lost. The Democrats threw away their best defense" by dropping plans for a filibuster.

But a Democratic Senate aide said Manion's opponents had made an impressive show of strength despite Gorton's vote switch and other questionable GOP tactics. The Republicans "lost the one thing that matters most in the Senate -- integrity," he said. "They lost the moral high ground, and they still don't have Mr. Manion."

The showdown reflected both the intensity of feeling and the political divisions in both parties over the Manion nomination. The battle over the conservative Indiana lawyer, who opponents say is too inexperienced and too extreme for the job, has escalated into a major test of Reagan's ability to win confirmation of his choices for the federal bench.

The Democrats were stunned when Gorton, saying he had been assured of a federal judgeship for a Seattle lawyer whom he has been pushing for months, voted for Manion's confirmation, depriving the anti-Manion forces of the majority they thought they had.

Gorton's defection left the Democrats with a 47-to-47 tie that Vice President Bush, presiding over the chamber, would have broken in Manion's favor. The Democrats avoided this outcome with a parliamentary maneuver that led to an agreement to postpone another up-or-down vote until next month.

When it became clear that a tie would result, Minority Leader Robert C. Byrd (D-W.Va.) voted for Manion. That gave Manion's supporters a 48-to-46 victory, but it also gave Byrd standing to move for a reconsideration of the vote. Both sides declared victory and agreed to put off the nomination until July.

Sen. Joseph R. Biden Jr. (D-Del.), who led the floor fight against Manion, said the vote "makes clear that the only way he can get on the bench is not on his merits, but on commitments made by the president on extraneous matters."

Biden criticized the vote switch without naming Gorton, saying: "We're talking about trading off a lifetime of bad judicial judgment for some immediate gain."

Gorton, who is seeking reelection, said he regards Manion's qualifications as "no more than marginal," but that he had made "no commitments" on how he would vote.

Gorton said the Justice Department has been delaying for months on his proposed nomination of Seattle attorney William Dwyer, a Democrat, for a district court judgeship in his state. Citing the administration's support for Manion, the choice of Indiana's two Republican senators, Gorton said he told Justice officials that he wanted equal treatment. "When I was assured that my concerns would receive the same consideration as those of Indiana, I voted in favor of Mr. Manion," he said.

A Justice spokesman declined comment. Sen. David F. Durenberger (R-Minn.) Tuesday dropped a threat to block a vote on Manion after the White House assured him it would approve a Minneapolis lawyer whom Durenberger has been pushing for a judgeship.

Democrats also cried foul yesterday over the loss of two other votes that could have defeated Manion's nomination:Sen. Nancy Landon Kassebaum (R-Kan.) voted against Manion, but switched her vote at the last minute after the GOP leadership told her that Sen. Barry Goldwater (R-Ariz.) wanted to support Manion but could not make it to the floor. Kassebaum said that was "a misunderstanding" and that she later learned that Goldwater had planned to vote "present" or not at all. Majority Leader Robert J. Dole (R-Kan.) said Sen. Bob Packwood (R-Ore.), one of two GOP members who were out of town, planned to vote for Manion, but Packwood's staff said later he was undecided. This was crucial because, in an effort to force a vote, two Democrats agreed to abstain to offset the absent Republicans on the assurance that both were Manion supporters.

Sen. Daniel J. Evans (R-Wash.), who had joined Gorton in recommending Dwyer for a judgeship, nevertheless voted against Manion.

Republicans joining Evans were Sens. Lowell P. Weicker Jr. (Conn.), Charles McC. Mathias Jr. (Md.) and Arlen Specter (Pa.). Democratic Sens. Howell Heflin (Ala.) and Russell B. Long (La.), however, sided with Manion's supporters.

The day began with both sides girding for a Republican cloture petition to limit debate on Manion, which would end Democratic plans to talk the nomination to death. Republicans repeatedly said it was unfair to filibuster a judicial nominee.

But Biden left Dole speechless by suddenly offering to vote up or down on Manion. He said he wanted to vote before the July 4th recess because "I don't want to find out there's been an Air Force base moved from Delaware to Maryland, or. . . the president takes some people on a helicopter ride."

Dole, after retreating to count noses, said he did not want "to roll the dice." But he agreed to the vote after Biden offered the two Democratic abstentions.