The last remaining hopes for a postponement of Nicaragua's Nov. 4 election and participation of the major opposition coalition in the vote disappeared this weekend as the government, the opposition and the principal outside mediator agreed there was no point in continuing the effort.

In a news conference here this morning after two days of talks, former West German chancellor Willy Brandt said he knew the effort was useless even before he arrived here Friday for a final try at bringing the two sides together.

However, Brandt, who represented the Socialist International as mediator in the talks, said he still considered the elections "a step forward for Nicaragua" even without the participation of the opposition Democratic Coordinator and its presidential candidate Arturo Cruz.

The collapse of the attempt to find common ground on electoral and other political questions between the ruling Sandinista Front and the Coordinator, a coalition of four political parties, two labor unions and a business association, was almost anticlimactic. The final announcement seemed to turn a page that allowed the country to concentrate on widespread speculation about what now is considered a more pressing political question: what the Sandinistas and the Reagan administration will do once both their elections have passed.

Most diplomats and other observers here reject Sandinista claims that the United States is planning direct military intervention. But they say they do believe that attacks by U.S.-supported rebels will continue and that the Reagan administration may take other measures, such as severely reducing U.S. trade with Nicaragua, to make matters very difficult for the Sandinistas.

Meanwhile, the Sandinistas are holding a series of meetings with leaders of other political parties that are participating in the elections, forging accords they say will give those parties meaningful roles in a multiparty political system after Nov. 4. The Sandinistas are expected to win easily in the elections for president, vice president and a 90-seat National Assembly.

Observers here say the Sandinistas will make certain concessions to the six other parties to stem Reagan administration claims that their system is undemocratic and to make it more difficult for Washington to move against them.

The last hopes for a postponement of the election apparently died yesterday when Cruz, Coordinator presidential candidate, emerged empty-handed from a meeting with Brandt.

The Coordinator had hoped that Brandt, who was one of the mediators in talks between Cruz and the Sandinistas earlier this month in Brazil, would be able to persuade the Sandinistas to reopen those negotiations. The two sides appeared close to an agreement in Rio de Janeiro on Oct. 2. Sources involved in the talks said Sandinista commander Bayardo Arce first accepted the terms but later went back on the agreement when other Sandinista leaders opposed a postponement that Cruz said was needed to consult with his colleagues in Managua.

Brandt said Cruz and other Coordinator leaders presented him with a statement accepting the terms laid out in Rio, but he said it was too late.

"The train left the station Oct. 3," he said.

He expressed hope that there would be a "national dialogue" including all the political forces in Nicaragua and told reporters he had been assured by the Sandinistas that the Coordinator would be invited to participate in such a dialogue after the elections.

Many observers here fear that President Reagan, if reelected, will order a sharp reduction in trade between the two countries. Seventeen to 20 percent of Nicaragua's trade is with the United States, according to government sources.

Nicaragua imports spare parts for vehicles and machinery and also buys fertilizers and herbicides vital to its agricultural economy from the United States. It exports meat, seafood products, bananas and other commodities to the United States.