A longstanding idea to launch a sort of Marshall Plan to aid Central America suddenly has become the newest bandwagon on Capitol Hill.

Its latest form is a proposal, introduced 3 1/2 weeks ago by Sens. Henry M. Jackson (D-Wash.) and Charles McC. Mathias Jr. (R-Md.), for a blue-ribbon study commission to decide overall U.S. policies in the region. It has attracted at least 25 Senate cosponsors, including 13 Republicans, and all six declared Democratic presidential contenders have endorsed it.

In the House, it is cosponsored by liberal Michael D. Barnes (D-Md.) and conservative Jack Kemp (R-N.Y.), who plan to call for backing this week from colleagues within their considerable ideological range as Congress returns from its holiday recess.

The Reagan administration has not endorsed the plan publicly but has left little doubt privately that a bipartisan congressional call for a study commission would be welcomed warmly at the White House. National security affairs adviser William P. Clark told Mathias the idea "was exactly the response they were looking for," according to a Mathias aide.

Under the proposal, the president would appoint a bipartisan national commission of distinguished business, Hispanic, labor, government, education and religious leaders. They would consult with Central American leaders and political figures about long-term problems of poverty, democratic development and human rights in the region.

The commission would report in six months, making recommendations on military and economic aid, trade, political and social policies the United States should pursue to help deal with the problems over the next 50 years.

Some critics have expressed concern that the commission offers only long-range answers to questions likely to explode in the short term. They say it would give confused members of Congress a way to avoid facing issues now.

Some liberals have said they fear that President Reagan would stack the commission membership in his favor, while some conservatives doubt the value of any foreign-aid plan. Many also think that any truly bipartisan group would be unpredictable, a loose cannon whose recommendations could backfire on Reagan.

Overall, however, there appears to be more enthusiasm than doubt. Hope springs mainly from previous commissions' success in forging workable solutions to two other tangled and stubborn problems: the Scowcroft commission on MX missile deployment and the Greenspan commission on long-term Social Security financing reforms.

While neither commission's report fully pleased anyone, both served as bases from which Congress ultimately reached at least temporary agreements.

Reagan's current Central America policy faces widespread skepticism and ignorance among the public and in Congress as election year approaches, but no coherent counterproposals for the entire region have emerged. The result has been bickering and stalemate over an issue that all sides agree is crucial to the United States' role in the world.

U.N. Ambassador Jeane J. Kirkpatrick began pushing the idea of a kind of Marshall Plan after returning from a February visit that left her impressed by the poverty and suffering of Central America's people.

"I alluded in some public places to the powerful needs of the region," she said in an interview, declining to claim responsibility for the idea, "but what happened was that a lot of people began to talk to me, saying they'd been thinking about this for a long time and wasn't it time to do something about it."

All sides, however, were aware that Congress perennially objects to foreign aid as a giveaway when Americans are unemployed. The Caribbean Basin Initiative, a program of trade and tax incentives launched with much fanfare last year, approaches final Senate approval this week in a version drastically pruned at the insistence of labor and business.

Jackson said he made his proposal in order to derail the increasing partisan tensions of current debate.

"I was in the House when a Republican Congress put together the Marshall plan with a Democratic president," Jackson said. "We laid the foundation for stability in Europe, and we did it on a bipartisan basis."

He said he is "not naive" about the differences between reconstructing Europe's shattered industrial economy for an educated, middle-class population of democratic traditions, and building a new economy for a semiliterate, poor agricultural people with a much more authoritarian history.

"We're dealing with the third world, but we have to make a beginning," he said.

Barnes, head of the House Foreign Affairs subcommittee on Western Hemisphere affairs, has visited the region frequently and agrees with the administration that vital U.S. interests are at stake there. He has been pushing his "1 percent solution," a diversion of 1 percent of the U.S. military budget, or $1.65 billion, into foreign aid for the region, and saw the commission idea as a way around partisan roadblocks, an aide said.

Kemp, a conservative backer of foreign aid, agreed immediately when Barnes asked for his support, one of Kemp's aides said. "He feels it's an important vehicle for highlighting our strategic concerns there," the aide added.

All agree that major problems such as these lie ahead:

* Composition of the commission. Lists are circulating, and "all kinds of people are already offering their services and advice," a Mathias aide said.

* Nicaragua. The Marxist-Leninist Sandinista government sits astride the Central American isthmus, its communications and economy integrated fully into the region. Any recommendations that include aid to Nicaragua would hit a firestorm in Congress, but any that ignore the nation would, too.

Jackson recalled that Marshall Plan aid was offered to Soviet bloc nations but turned down by Soviet leader Josef Stalin. "We don't rule anything out," Jackson said.

* Money. The first four years of the Marshall Plan put $13.2 billion into 16 European countries, or 1.1 percent of the U.S. gross national product over those years. An equivalent amount today would be $160 billion. That and more could probably be spent usefully in Central America, with its greater problems, over the next 50 years.

Balancing the useful with the possible "is what politics is all about," a staff aide said. Jackson noted that Mexico alone owes $90 billion to the world banking community and that "a default would cost us all far more than that."

* Development ideas. Any commission choice among dozens of competing theories of economic development is sure to cause controversy here and in target areas. Any recommendations for population-control efforts are expected to spark conservative opposition, while liberals would oppose a call for more military help.

Labor-union development, agricultural reform, tax-law rewriting, media freedom and political party structure, all development steps, are highly charged issues in Central America, while some conservatives argue that all aid is useless until fighting stops.

The Senate Foreign Relations Committee plans hearings July 27 on the issue, and Jackson said he hopes the Senate can pass the resolution to establish the commission before the August recess. "I think we might almost get it by unanimous consent," he said.