The one new element in the President Reagan's recent moves on Nicaragua is Arturo Cruz, by far the most respectable and plausible figure among the anti-Sandinistas.

Cruz, 61, is a Nicaraguan banker who was educated at Georgetown University. He was a member of the first post-Somoza junta and the Sandinistas' ambassador to the United States but quit the government in protest in 1981. He went back to Managua to run for president in last November's election but withdrew before the balloting.

His value to Reagan, who is facing an uphill battle in his strenuous campaign to win $14 million more for the "contras," the force organized and funded by the Central Intelligence Agency to overthrow the government, is that Cruz was once a seemingly implacable enemy of the contras.

In a 1983 issue of Foreign Affairs, Cruz called the Nicaraguan revolution "irreversible" and warned that a military victory by the contras, who are led by former members of the Somozista National Guard, could lead to a bloodbath and the "mass execution of the flower of our youth."

To Cruz, then, the contra leaders were "civic cadavers" and their backers, wealthy Nicaraguan exiles, "the Key Biscayne Mafia."

But Cruz has changed his views. In a speech to the Woman's National Democratic Club last Thursday, he never mentioned the CIA, called the Sandinista regime a "disguised dictatorship" and endorsed any military and economic options the United States might use against it in the name of peace. He wants "a dialogue of reconciliation" between the contras and the Sandinistas. Nicaragua, he said, should no longer be "divided between angels and devils." He elevated the terrorist raids of the contras to the dignity of "civil war."

He said, in short, just about what the president said an hour later in a briefing on his "peace" initiative. The offer sounded suspiciously like one the Sandinistas were bound to refuse, as indeed they did. The Sandinistas seem as unlikely to bow to Reagan's demands for a new election and talks with the contras as the Soviets are to accept "Star Wars" as a wonderful way to peace.

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) promptly labeled the whole thing a "dirty trick." But critics of the covert war, who hate to see Uncle Sam bullying a small country and are repelled by contra atrocities, are getting nervous. In the House, their 20-vote margin begins to look shaky. A strong-minded president talking about peace is a formidable foe. Only the bravest will say a word for the Sandinistas or question the president's premise that he has a perfect right to practice unlimited "behavior modification" in a small, peasant nation.

In the Senate there are few Republicans who can resist the chance to vote for "peace," against "communism" and with Reagan. They know that if the Sandinistas still say no after the 60-day deadline proposed by Reagan expires, the $14 million, which would go to humanitarian uses in the meantime, would go right back to funding the war -- which many of them profess to deplore.

After his briefing, the president met with Cruz and two other contra supporters, Adolfo Calero and Alfonso Robello.

After Cruz's aborted candidacy in Nicaragua's election last November, he said the United States should give President Daniel Ortega a chance. He has since changed his mind. In his speech last week, anxious not to be branded a sore loser, Cruz said, "I did not boycott the election; I was excluded." He said his rallies had been disrupted.

The Sandinistas claim that Cruz invited disruption by advocating negotiations with the contras, a provocation to Nicaraguan voters whose relatives have been killed or kidnaped by the mercenaries. Cruz, however, says it is because the Sandinistas fear true democracy.

He is also self-conscious about being tagged as Reagan's man. At the Democratic lunch, he began by insisting that he was "not lobbying for proposals from any branch of your government."

He sounded as if he were. While he did not go so far as to ratify Reagan's comparison of the contras to the Founding Fathers or Secretary of State George P. Shultz's characterization of Nicaragua as "a Soviet state" -- Cruz said it is a "disguised dictatorship" -- he seems to feel the contras are a splendid group, worthy of a full role in Nicaraguan society.

Paul Reichler, the young Washington lawyer who represents Nicaragua, says that Cruz is being used by the contras.

"He's dreaming of becoming president of Nicaragua," Reichler says, "and he thinks the contras will help him. The day that Congress gives them the $14 million -- if it does -- Cruz is finished. They won't need him any more."

Meanwhile Cruz, the only new voice in the administration's extraordinary effort to make the Sandinistas say "uncle," is going to be extremely useful to the president.