President Reagan, pledging that the United States must continue financial aid to the Nicaraguan contras, said yesterday that the Sandinista government has made "a good beginning" toward ending the conflict in Central America but has not moved far enough.

"We'll not be satisfied with mere show -- with Potemkin reforms," the president said, referring to the scheme of a Russian statesman to erect sham villages on the shore of a river to impress the empress of Russia in 1787.

In what was one of his most strident speeches since he and House Speaker Jim Wright (D-Tex.) announced a joint peace initiative for Central America Aug. 5, the president said "the real problem" is that "communist Nicaragua is, in fact, a Soviet beachhead in the Americas."

For this reason, he told 1,500 conservative women at the Crystal Gateway Hotel in Arlington, U.S. aid for the rebel forces "must and will continue."

The president did not say that he was backing away from his agreement to give the peace process time, but he spoke of "inevitable complications" in the peace process set in motion last month by five Central American presidents and backed by his administration.

Rep. Tony Coelho (D-Calif.), the House majority whip, said last night he saw the president's address at the Concerned Women for America convention as an effort to push the government of Nicaraguan President Daniel Ortega into breaking off from the peace process.

"The president won't listen to anyone," Coelho said. "In order to move forward with his negotiations with the Russians, he thinks he has to bash the Nicaraguans to appease the right wing" of the Republican Party. "If he would spend as much time trying to help the peace process as he spends bashing it, we would would have peace down there."

White House aides said the speech was not a departure from that policy but an attempt to signal the Sandinista government that Reagan expects more concessions from them and expects them more quickly.

The negative rhetoric from Reagan and other administration officials about the process has troubled Wright and others on Capitol Hill who say they fear Reagan may be attempting to brand the peace process as a failure.

"There's only one fair path to peace: free and fair elections open to all," Reagan said to sustained applause by the convention participants. He hailed moves to eliminate censorship in Nicaragua, steps that have allowed an opposition newspaper to resume publication and a Catholic radio station to resume broadcasts.

"But it's only a beginning," he said, ticking off a list of "obvious" actions he would like to see the Nicaraguan government take.

" . . . Open up the jails and let the thousands of political prisoners free; let the exiles come home; allow freedom of worship, free labor unions, a free economy; dissolve the so-called 'neighborhood watch committees,' and give the people of Nicaragua back their basic human rights," he said.

"And last, but not least, send the Soviets and Cubans home."

To demand anything less, Reagan said, would not serve the cause of peace in Central America. "Until those conditions are met, 'democratization' will be no more than a fraud. And until they're met, we'll press for democracy by supporting those who are fighting for it."

The audience of the eight-year-old women's organization, which describes itself as the nation's largest women's group, interrupted the president's 20-minute address every 30 seconds on the average, cheering his support of the contras as lustily as they championed his support for Supreme Court nominee Robert H. Bork.

If the cheers for the contras were slightly louder, according to convention participant Larry Little, a Republican from Randolph County, N.C., it was only because that cause is more established than the Bork confirmation. The group feels strongly about the contras, he said, because they see the communist threat as real and fear the spread of communist government "will move right up from Mexico to the U.S."

The support for the two causes was reflected in the sales of political buttons at the convention. Button salesman Frank Enten said sales of pro-Bork buttons edged ahead of those supporting Marine Lt. Col. Oliver L. North, the White House aide fired for his secret efforts to divert funds from Iranian arms sales to aid the contras.