The National Republican Congressional Committee has attacked Democratic opponents of a conservative-sponsored amendment to the International Monetary Fund authorization bill as supporters of communism, and members of both parties say that could jeopardize approval of a bill strongly supported by the White House.

The GOP campaign group sent news releases this month to the media and Democratic contributors in about 20 congressional districts held by Democrats, condemning members opposing an amendment sponsored by Rep. Phil Gramm (R-Tex.). It prohibits the United States from supporting IMF loans to nations run by "communist dictatorships."

"What gives them the right to support communism?" Gramm asked in the news release.

The amendment, passed by a 242-to-185 vote, was opposed by the administration. Among those joining the 20 targeted Democrats in opposition were House Minority Leader Robert H. Michel (R-Ill.) and ranking Republicans on the House Banking, Finance and Urban Affairs Committee and the Ways and Means Committee.

The $8.4 billion IMF authorization bill was approved by six votes later that day, Aug. 3, after heavy lobbying by the administration.

The next day, the NRCC sent out news releases saying Democrats had "voted . . . to loan U.S. taxpayers' money to communist nations."

"It's a slap in the face of the taxpayers of this country that member's name would vote to give loans to communist dictatorships," Gramm said in the release.

Rep. Tony Coelho (D-Calif.), chairman of the House Democratic Congressional Campaign Committee, said yesterday that the release was "vicious, nasty, mean, probably the lowest level of politics I've seen in a long, long time."

"I think you could easily be concerned about whether or not this is a resurrection of the McCarthyism-type activities," he said.

Coelho predicted that the release could doom the bill. Of the 20 Democrats known to have been targeted with the letter, 13 supported the administration on final passage. If three switch positions, the bill would not pass.

"It's either a vicious tactic or a major blunder," Coelho said.

The House must vote again on the bill when it clears a House-Senate conference, and a separate appropriations bill must be approved.

"It may cause some problems legislatively in maintaining support for the administration position when we go into the appropriation position here," said Michael Johnson, Michel's press secretary.

Administration officials are privately concerned about the potential impact on the IMF bill but have not protested to the NRCC. Johnson said that Michel "wasn't overjoyed" by the campaign committee's tactic but that the minority leader would not condemn it.

"We have not heard from the White House, and it's fair to assume that if we were causing grave difficulties for anyone else in the leadership or the White House, we would know about it," said NRCC spokesman Steve Lotterer, who denied any effort to make anti-communism a theme of the 1984 campaign.

Lotterer, saying it is "not our practice to clear releases," added that it was one of a series sent out by the committee this year as part of its strategy to soften up Democratic incumbents for the 1984 elections.

"If anyone tries to say that a small targeted press release is going to jeopardize a piece of legislation, it may mean the bill was on shaky grounds to begin with," he said.

Asked why the release was not sent to Michel's district, Lotterer said, "We're in business to elect Republicans."

The Gramm amendment would have no effect on the Soviet Union, Cuba, East Germany, Poland and Bulgaria, which are not members of the IMF.