The House in effect declared limited, surrogate war on Nicaragua's leftist Sandinista government last week, voting to give President Reagan the funding, the tools and the political support he has been seeking for four years.

The 221-to-209 decision, almost certain to be ratified by the Senate next month, was appropriately narrow for an issue that has split the nation's policymakers as have few others since Vietnam. But it came cleanly, after bitter, lengthy and excruciatingly detailed debate, and it provided a wholesale commitment.

No one contends that $100 million in new aid will give the counterrevolutionaries, or contras, military victory over the Sandinistas or bring peace to the region. The question is whether the House vote will be the turning point that ends the fundamental policy debate, as did Congress' 1984 decision to aid President Jose Napoleon Duarte of nearby El Salvador.

U.S. interest in El Salvador now focuses on whether the policy is working, rather than on what the policy should be. Whether that will be the case in Nicaragua appears to hang on the contras' military performance after military aid starts Sept. 1.

The House agreed that Reagan may use the Central Intelligence Agency, the Defense Department and any other agency he pleases to provide the contras with $70 million in arms, training, transport and advice and $30 million in food, clothing and medicine. Democratic nations in the region will receive $300 million in economic aid, and all of it will be subject to strict -- but confidential -- accounting practices.

There were all-night parties in the rebels' jungle camps, while squabbling contra factions here found common cause and embraced. With Congress committed, they felt themselves becoming an irresistible force. In a year, "we will be seeing cracks in the Sandinista structure," predicted Alfonso Robelo, one of three top contra leaders.

The Sandinistas responded by swearing to become an immovable object. They retaliated, closing down the only critical newspaper and vowing redoubled defiance to U.S. demands. "To war you respond with war," President Daniel Ortega declared.

Robert S. Leiken is a senior associate at the Carnegie Endowment for International Peace and a former opponent of contra aid. He analyzed the mood in Congress as one of hopeful willingness to give Reagan and the contras a chance to fulfill their promises, rather than of certainty that this vote will change history.

"It is a step towards settling the policy issue, but it isn't settled yet," he said. "Congress had a great deal of confidence in Duarte and there isn't the same confidence in the contra leadership."

To win, administration officials had to overcome critics' charges that the contras were puppets of the CIA, hopelessly divided, inept and leaderless; that they were corrupt and involved in drug traffic, and that they had committed major human rights violations that cost them popular support.

After lengthy negotiations, contra leaders announced unity and said their ranks were open to all, such as Eden (Commander Zero) Pastora, who have so far refused to join. They promised to hold a leadership selection convention.

The administration issued a point-by-point rebuttal of House Foreign Affairs subcommittee charges that earlier aid might have been diverted, and Senate Republican leaders promised a vote on whether to use subpoenas to probe other, more serious allegations of drug trafficking and gun-running.

The contras set up their own human rights office. The new aid will provide funds for that office, a congressional bipartisan monitoring commission, a six-member accounting team and military training that stresses human rights. Rebel leaders insisted they welcome all congressional investigations and will be open to media scrutiny, and they said that despite the lack of U.S. military aid for more than two years, their ranks have swelled to nearly 20,000.

The administration also had to deal with U.S. public opinion polls that show 62 percent opposed to contra aid and supporting negotiations instead.

Two weeks before the House vote, the Contadora peace talks, approaching and withdrawing from a treaty agreement with tide-like regularity, broke down spectacularly. Latin democratic nations, previously opposed or silent on contra aid, began insisting that Nicaragua establish democratic procedures as part of any treaty.

Lobbyists then reminded House members that polls also show the majority of voters ignorant of Central America and not voting on that issue. Why debate the question in Congress again next October, just before the elections, as the Democrats' alternative bill would have required?

All sides acknowledge that Reagan's House victory may be transitory. Additional aid -- which no one denies will be requested -- will hinge on whether the contras clean up their act and make military progress, and whether that in turn pushes Nicaragua toward democratic reforms.

"Believe me, you will face this issue again and again as long as you're in the House," Minority Leader Robert H. Michel (R-Ill.) told party colleagues the morning of the vote. "You'd better understand that and join now, because this isn't going to go away."