Secretary of State George P. Shultz, asserting a need to "end the doubt and uncertainty" about U.S. support for Nicaragua's contras, said yesterday that President Reagan plans to ask Congress for $270 million in new military and other aid for them over the next 18 months.

In testimony before the Senate Foreign Relations Committee, Shultz said the request will not be made until after the contras' $100 million in U.S. aid runs out Sept. 30. But, in the face of repeated calls by Democratic members of the committee to hold the request until peace talks among five Central American countries are concluded, he refused to say when the administration is likely to act or whether it actually will spend the money for military purposes if Congress approves the aid.

Shultz's arguments ran into a seemingly solid wall of resistance from the Democratic majority on the committee and from House Speaker Jim Wright (D-Tex.), who joined Reagan last month in sponsoring a bipartisan peace plan for the region. Their action prompted the five Central American governments to adopt a separate initiative proposed by Costa Rican President Oscar Arias.

The administration has taken a cautious approach toward the Arias plan because of concern that it contains insufficient guarantees that Nicaragua will permit greater internal democracy and cut its ties with the communist bloc. Wright, though, has backed the Arias initiative enthusiastically, and he told reporters yesterday that it would be "inappropriate" to seek aid for the contras "during the time the peace process is moving forward . . . . If such a request is made it would anticipate the failure of the peace plan. I don't anticipate failure."

Shultz denied that sending the request to Congress before Nov. 7, the deadline set by the Central American governments for reaching agreement, would jeopardize chances for a regional peace accord. Instead, he insisted that enabling the contras to maintain their military strength would put pressure on Nicaragua's Sandinista government to stop trying to spread communist subversion in the region and resolve its disputes with its neighbors.

While insisting that the administration supports the negotiations by Nicaragua, Costa Rica, El Salvador, Honduras and Guatemala, Shultz added:

"It is simply not in our national interest to leave the Sandinista regime unconstrained by credible resistance forces on the basis of a hope or a premise. We have too much at stake. This president will not stand idly by -- this secretary of state will not stand by -- and permit a country as near to our borders as Nicaragua to become a place from which the Soviet Union and its allies can militarily threaten our friends or our country's national security."

Aides to Wright said that on Wednesday night Reagan's national security adviser, Frank C. Carlucci, had told the speaker and the rest of the House Democratic leadership of the administration's intention to submit the $270 million contra aid request Oct. 1. According to the aides, Wright and the others told Carlucci that such a move would cause Congress to vote overwhelmingly against the request. That warning apparently caused the administration to revise its strategy and adopt the more flexible approach taken by Shultz yesterday.

While saying that the Central American negotiations contain "many positive and hopeful aspects" that the United States wants to pursue, Shultz added, "We need to end the doubt and uncertainty about the capacity and commitment of the United States {to the contras} that is created by the recurring cycle of off-again, on-again aid decisions punctuated by protracted and divisive debate."

However, committee members who oppose contra aid said the administration's move is a "wrong signal" that threatens to undermine the peace talks by allowing Nicaragua to charge the United States with continuing aggression. Carlos Tunnerman, the Nicaraguan ambassador here, said Shultz's contra-aid announcement "shows contempt for the leaders of the region who called upon the United States to stop waging war against Nicaragua."

Shultz also said that Reagan has named Morris Busby, a deputy assistant secretary of state, to be a roving ambassador to coordinate U.S. diplomatic contacts with the Central Americans. Busby will take over at a lower level some of the functions that were performed by Philip C. Habib, who resigned last month as Reagan's special envoy for Central America following a dispute over negotiating strategy.

Sen. Jesse Helms (R-N.C.), a staunch conservative who is seeking to attach an amendment for contra aid to proposed legislation on limiting campaign spending, changed his original request for $310 million to conform to the administration's proposed $270 million figure. The Senate took no action on Helms' amendment yesterday.