WHITE SULPHUR SPRINGS, W.VA., JAN. 22 -- Nineteen House Democrats, most of them moderate members of a bloc of swing votes that will determine the fate of further U.S. aid to the Nicaraguan contras, today called on President Reagan to delay his request for new assistance to the rebels.

In a letter to be sent to the president four days before the administration is expected to present its request for as much as $50 million in new aid, the Democrats urged a postponement to keep pressure on Nicaragua's Marxist Sandinista government to comply with the five-nation Central American peace plan signed in Guatemala Aug. 7.

The letter also urged Reagan, if he does not postpone his aid request, to limit it to nonlethal assistance, a strong indication that the administration's plan to include a small percentage of military aid in the package may be defeated when the House votes on Feb. 3.

Democratic leaders greeted the proposal coolly. House Speaker Jim Wright (D-Tex.) said he had not "had a chance to think about it."

House Democratic Whip Tony Coelho (D-Calif.) said that Congress and the administration should abide by the compromise forged late last year calling for the vote on Feb. 3 in the House and the next day in the Senate. Under that plan, if Reagan loses, he would have no further opportunity for a guaranteed congressional vote on the issue, although he could seek a vote on additional aid through regular legislative channels that could more easily be blocked by the Democratic leadership.

In a campaign appearance in Iowa Thursday, Senate Minority Leader Robert J. Dole (R-Kan.) indicated willingness to delay the vote to see what happens in Central America. "It seems to me that if we could work it out, we ought to postpone the vote," he said.

"We view any move to advance new U.S. aid to the rebels as extremely risky and a threat to lasting peace in Central America," the 19 Democrats said in their letter. "The U.S. must not be the obstacle to peace in Central America."

The letter was written by Rep. Dave McCurdy (D-Okla.), who two years ago spearheaded an unsuccessful effort by moderate and conservative Democrats to find a compromise between Reagan's request for $100 million in contra aid and the Democratic leadership's efforts to defeat it. Reagan won that battle as Congress narrowly approved the package that included $70 million in military aid.

McCurdy, widely viewed as an influential voice among the 50 or so moderates from both parties who will determine the outcome of the scheduled Feb. 3 vote on Reagan's new request, disclosed the letter as he and about 130 colleagues traveled to a meeting of House Democrats at the Greenbrier resort here. He said:

"The problem with a vote right now, regardless of the outcome, is we could let {Nicaraguan President Daniel Ortega} off the hook. If the critics {of contra aid} vote it down, it takes the pressure off. If we vote it in, and it looks like too much, he {Ortega} can say Reagan is the problem."

"We would prefer a date certain," said Coelho, the Democrats' chief vote-counter in the House and an outspoken opponent of contra aid. "Let's go ahead and make a decision . . . . What they are doing is suggesting a way for the president to get off the hook."

In Washington, Reagan, continuing his fight to marshal backing for the aid request, told a group of supporters gathered at the White House that the upcoming congressional vote will determine whether there is a chance for democracy in Nicaragua.

"We must make sure that every time the Sandinistas walk through a new door to democracy we close it behind them and keep it closed," he said. "Only the freedom fighters can do that."

However, Reagan also reiterated that if a cease-fire is achieved between the contras and the Sandinistas and progress is made toward a political settlement in Nicaragua, the United States would agree to take part in "regional security discussions."

That offer, first made last November, was in response to Ortega's call for direct negotiations with the United States. Administration officials insist that such talks should be in "a regional context" involving all five countries in the peace plan.