The Senate last night approved a newly watered-down version of President Reagan's proposal for $14 million in aid to antigovernment rebels in Nicaragua, 53 to 46, but soon afterward the House rejected the same plan, 248 to 180.

The House is to vote on alternatives to the Reagan plan today and, if one is passed, House and Senate conferees would try to agree on a compromise aid package.

The seven-vote margin of support for the president was the smallest the Republican-controlled Senate has given him on aid to the rebels, and the House margin of defeat was four votes larger than any previous House rejection of Reagan's three-year-old aid program for the "contras."

The White House released a statement from the president warmly praising the Senate for casting "an historic vote" for "a consistent and effective policy that is true both to our principles and to our interests."

But, before the Senate voted, Reagan had further modified his plan in an unexpectedly conciliatory letter promising to renew direct U.S. negotiations with the leftist Sandinista government in Nicaragua. The United States broke off the talks in January.

Reagan's letter, delivered to Senate Majority Leader Robert J. Dole (R-Kan.) just 75 minutes before the final Senate vote, also pledged to "press for" a cease-fire between the rebels and Sandinistas. The original Reagan plan would not have put any pressure on the rebels to negotiate or reach a cease-fire.

As expected, the letter also promised that the $14 million would be spent "only for food, medicine, clothing and other assistance for their [the rebels'] survival and well-being -- and not for arms, ammunition and weapons of war."

In addition, according to a senior administration official, Reagan has decided to administer the $14 million aid package to the rebels through an interagency committee that he will chair. This means that the aid would not be controlled by, but handled by, the Central Intelligence Agency, as the administration originally wanted.

Reagan's letter also promised to "favorably consider" economic sanctions against Nicaragua and to open multilateral consultations with other Central American nations on such sanctions. This course was favored by Sen. Lloyd Bentsen (D-Tex.), one of 10 Democrats voting last night for the watered-down Reagan plan.

"We will use our assistance to help ensure against wrongful acts by those who seek our help," the letter added, apparently responding to charges of rebel atrocities.

"This letter concedes most of what we were trying to get from the president in the form of legislation," Minority Leader Robert C. Byrd (D-W.Va.) said. "This is no way to legislate."

The conflicting House and Senate verdicts set the stage for decisive House votes today on two alternatives, one from Republicans that closely follows the Senate-approved measure and one from Democrats that provides general humanitarian aid to the region but not the rebels.

Either would be far less than the wholehearted endorsement for the rebel cause that Reagan has demanded in several weeks of emotional speeches but more than Democrats had been willing to provide the insurgents. Democratic leaders said they expect a much narrower vote today.

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) said last night's House vote was a clear signal from members of concern about potential U.S. involvement in Nicaragua. "They don't want our boys down there. That's what it's all about," he said.

The Senate vote was rather a grudging one, awarded by conservative Democrats who complained that White House rejection of their weekend compromise offer forced them to take the lesser of two evils: supporting the president rather than abandoning the rebels altogether.

The Democrats' proposal, worked out Sunday by eight senators who ordinarily seldom agree, would have provided the rebels $14 million in humanitarian aid through a government agency other than the CIA and would have offered more funds to implement any later peace agreement.

Dole said that proposal "would have taken away the president's flexibility." He called the letter a "carefully crafted effort to reach bipartisan support."

Sen. J. Bennett Johnston (D-La.) had called the Democrats' offer "a minor miracle" and said it was "a proposal the administration could not refuse, and yet the administration did refuse it."

Although he called that refusal "a serious mistake," Johnston said he would "very reluctantly and with reservations" back Reagan instead of the Democratic leadership. "I am personally not willing to walk away and abandon the contras," he said.

Sen. David L. Boren (D-Okla.) cited similar views and Reagan's letter as reasons for his support of Reagan. "The president has gone more than halfway in meeting the suggestions raised in the last few days by those of us on the other side of the aisle," he said.

Senate intelligence committee Chairman David F. Durenberger (R-Minn.) said he recognized that a letter "is not policy" but voted with Reagan because "at least it is conditions" on the assistance.

Sen. John Glenn (D-Ohio) disagreed, saying, "We're not voting on letters. This is a blank check for a Gulf of Tonkin resolution," referring to the vote that launched the U.S. military buildup in Vietnam.

Critics said that, had the CIA funneled the aid, the rebels could have used it to purchase military equipment. Dole later told the Senate that he expects Reagan to name the National Security Council as controlling body, a reference to the interagency committee that the White House now plans to use to administer the aid.

Sen. Barry Goldwater (R-Ariz.) charged that it was "wrong, wrong, wrong" of two junior Democratic senators, Tom Harkin of Iowa and John Kerry of Massachusetts, to visit Nicaragua last week and return with a negotiating proposal from President Daniel Ortega.

Kerry responded by reading a letter from Secretary of State George P. Shultz endorsing the idea of congressional visits to all Central American nations, including Nicaragua.

In the Democratic-controlled House, the debate followed traditional ideological lines, with liberals and moderates opposing further aid to the rebels, whom they termed uncontrolled "thugs," and more conservative lawmakers supporting the aid and calling the contras "freedom fighters."

Opponents of further aid said a diplomatic solution is necessary, accused the rebels of having committed atrocities and warned that the Reagan administration would increase direct U.S. involvement if funds were not cut off.

House intelligence committee Chairman Lee H. Hamilton (D-Ind.) called the $14 million "a down payment on greater military involvement" and "the next step on the slippery slope to further major U.S. military involvement in Nicaragua."

He charged that administration support of the rebels has left the United States and the CIA, which ran the rebel program, being "tarred with contra atrocities." While those probably have been no worse than similar ones by government forces, he said, "the difference is the United States is funding the contras."

But House Minority Leader Robert H. Michel (R-Ill.) said, "The issue here is do you want to help the forces of democratic pluralism in Nicaragua, or do you want to consolidate the power of a Marxist-Leninist dictatorship?" He said giving up on the rebels would be "de facto handing over of Nicaragua to the allies of the Soviet Union."

Despite the often heated rhetoric on both sides, yesterday's debate was not as emotional as in the past, mostly because the outcome of the vote was widely expected.

Republican leaders said Reagan's assurances in his recent "peace" initiative that the money would go only for "humanitarian" purposes had not persuaded a majority of House members to free the funds because his proposal would have allowed the money to be used for military purposes after 60 days.

House members had voted three times in two years against further military aid to the rebels. The last vote was 241 to 177 in May.

Michel (R-Ill.) said the House was being forced unfairly to vote yesterday on a proposal that did not reflect what Reagan is requesting. He accused O'Neill, a staunch opponent of the rebels, of forcing the vote solely to embarrass Reagan and win a victory over the White House after several major legislative defeats.

Democrats said they were only following an agreement worked out with the White House last year that provided the administration an up-or-down vote on the $14 million. But, they said, a solid defeat for the $14 million would send a clear signal that the House will not support further military aid.

Yesterday's debate was notable for one point rarely made. In the past, the rebels' supporters, including the White House, have argued that the rebels needed support because they helped to stop the flow of arms from the Sandinista government to leftist rebels in El Salvador.

Yesterday, lawmakers said, the rebels needed support because they would prevent the Sandinistas from solidifying their base of power.

"This is truly a broad-based revolutionary movement, and it deserves the support of the United States," Rep. Dick Cheney (R-Wyo.) said.