House Democrats seeking to curb President Reagan's policies in Central America said yesterday that their effort has lost its steam in the chill of new crises over the Middle East and the shooting down of the South Korean jetliner.

They said congressional attention has shifted away from the nation's troubled southern neighbors after reaching a peak six weeks ago with a decisive 228-to-195 House vote to cut off all covert U.S. aid to forces attacking the leftist Sandinista government of Nicaragua.

"Now people couldn't care less. They're saying we made our point in July, and the climate has changed in one of those strange ways," said an aide to Rep. Michael D. Barnes (D-Md.), who heads the House Foreign Affairs subcommittee on Western Hemisphere Affairs. "The feeling is that we can't do anything constructive in Central America because it would encourage the Soviet Union to shoot down more Korean planes."

The House legislation, called the Boland-Zablocki amendment, stands no chance of being considered in the Republican-controlled Senate, so the House would have to attach similar language to some other bill that could pass Congress and that Reagan would be reluctant to veto.

Backers of the legislation in both houses said they still are determined to raise the issue, and one vehicle could be the bill funding intelligence activities in fiscal 1984.

Boland-Zablocki supporters said they worry that House support for the aid cutoff might have waned.

"The longer the intelligence authorization vote is from the time the airplane crash happened, the more chance there is to gain perspective on it," said Rep. Clarence D. Long (D-Md.), who has led the fight to hold down increases in Reagan administration aid to El Salvador. Leaders who want to retain the earlier vote majority "would be wise to postpone it," he said.

No one is sure about the vote count, however, because the usual chitchat on such shifts is absent. "A lot of issues went down with that plane," another Foreign Affairs Committee staff member said.

Adolfo Calero, a leader of the rebel Democratic Nicaraguan Forces (FDN) who made the congressional rounds last week lobbying against the aid cutoff, said he had seen a real change in Hill sentiment. Where in June some congressmen criticized his movement, he said, "Now the main question is, 'Can you win?' or 'When can you win?' "

Rep. Dante B. Fascell (D-Fla.), second-ranking Democrat on the Foreign Affairs Committee, who did not support the aid cutoff, said, "It was the burning issue, but today everybody's saying, 'Where's Central America?' and the answer is, it's behind Lebanon and behind Korean Air Lines Flight 007. It might harden some votes, but you don't know. It's totally quiet."

Rep. Lee H. Hamilton (D-Ind.), a leader in the drive to end covert aid, said no members had informed him of any plans to change their minds, although he had heard that others were worried about the possibility.

"It's all highly speculative at this point," he said. The intelligence authorization bill is not expected to come up before mid-October, he said.

An aide to Rep. Edward P. Boland (D-Mass.) said he has no plans to bring up the aid cutoff at what would be the earliest possible opportunity, the House debate on a continuing resolution that will keep the government running in the absence of an appropriation bill when the fiscal year ends Sept. 30.

"It's only for 45 days. It would be a Pyrrhic victory," he said. He said there is "absolutely no chance" that the measure will be postponed indefinitely, however. "It's just a question of the vehicle."

On the Senate side, an aide to Sen. Edward M. Kennedy (D-Mass.) said that he and Sen. Claiborne Pell (D-R.I.) are still planning to force a floor vote on the Boland-Zablocki language, but that the timing is uncertain. "The Senate ought to be accountable on this issue just as the House has been," the aide said, "but there's no rush, as long as it happens before the session ends."

An aide to Sen. Daniel Patrick Moynihan (D-N.Y.), vice chairman of the Senate Intelligence Committee, said that all sides are lying low for awhile, but that the issue will resurface soon.

"There'll be fireworks later on" over aid to Nicaraguan rebels, he said. "You can make a case for doing more or for doing nothing, but it's hard to make a case for just diddling along like we're doing now."