The Pennsylvania House of Representatives comfounded previous predictions yesterday by rejecting the proposed constitutional amendment to give the District of Columbia full voting representation in Congress.

Legislative leaders had been confident of passage right up to the moment when the ballots were cast.

"I thought we had the votes," said Rep. James P. Ritter (D-Allentown) chairman of the committee that had held statewide hearings on the amendment, and who was one of its most ardent supporters here. But in the end, the proratification forces had only 89 votes to the opposition's 105.

Yesterday's action leaves New Jersey as the only state to have ratified the proposal, which passed the U.S. Senate on Aug. 22 and needs a total of 38 states to become part of the Constitution.

Democratic House Speaker Leroy K. Irvis, a sponsor and prime mover of ratification, attributed the defeat to what he called "political considerations" on the part of Pennsylvania Republicans, although he said other factors, including race, gun control and rural fears of big-city domination of Congress, had contributed to the rejection.

One of the amendment's more outspoken foes here. 22-year veteran Rep. H. Jack Seltzer (R-Palmyra) said he opposes the measure because "it is potentially harmful to Pennsylvania." Seltzer said he fears that if D. C. gets congressional representation the Keystone state could lose two of its own House seats after reapportionment based on the 1900 census.

Leaders of the nationwide campaign for ratification contended that yesterday's setback would not necessarily have a negative impact on the amendment's fate in other states.

Elena Hess of the Self-Determination for D. C. Coalition said that the vote "indicates what we know, that people don't understand Washington, D.C. They think of it as the federal bureaucracy."

Paul Hays, chairman of the D.C. Republican Party, criticized the tacties of D.C. Del. Walter E. Fauntroy, who has been traveling around the country lobbying for the amendment's passage.

"We may be able to turn this around," said Hays, "but it won't happen by flitting around day to day from Harrisburg to Columbus to Boize. We're going to have to sit down and educate legislators before bringing this thing up to a vote."

Fauntroy was in Columbus yesterday, testifying before the Ohio legislature, which may be the next to vote on ratification. He could not be reached for comment.

The Pennsylvania action, by a lame-duck Democractic controlled legislature, could well mean that the proposal is dead in the state until at least 1981. All but two of the House's 83 Republicans voted against it, and when the newly elected legislature convenes in January, Republicans, who made huge gains at the polls last week, will either control the House or be within a single vote of control.

Republican Governor-elect Richard Thornburgh has endorsed the amendment, but some observers here questioned whether he would be willing to fight next year for a measure that most legislators believe to be of little interest and less political benefit to their constituents.

"I think it's dead," said Irvins, studying a printout of the vote. Noting that 24 Democrats had voted against ratification, Irvins said "some of those votes puzzled me."

The controlling factor in the defeat, he concluded was Republican fear that D.C. representatives and senators would be Democrats.

"Sportsmen also entered into this," the speaker said. "Some of the hunters and fishermen told me they had been warned that passage would mean two more votes in the Senate for gun control."

Irvins said he also detected "some honest opposition from farmers" who fear D.C. representatives would favor urban interests over rural ones. And "race was very definitely a factor, as it is in almost everything that happens in this country," said Irvins.

He conceded that he and other black legislators in Pennsylvania had been "hoping to get a couple of replacements" for Sen. Edward Brooke (R-Mass.), the lone black in the Senate who was defeated last week.

In September, following a pattern set by California and Delaware, the Pennsylvania House resisted a move to bring the amendment to a quick vote and sent it back to committee for public hearings.

Only about 30 witnesses testified at the five hearings that were held, and when the matter came up for a vote in the House yesterday, there was no debate whatsoever.