The three major television networks yesterday agreed not to use exit polls to project the probable presidential-election winner in any state until that state's polls have closed.

The agreement opens the way to a possible uniform poll-closing time for presidential elections in the lower 48 states, according to two congressmen who have worked with the networks to prevent early election calls from influencing the election in states where the polls remain open.

The network agreement follows public criticism and pressure arising from early projections of the 1980 and 1984 presidential election winners based on poll data.

In 1984, the networks projected President Reagan as the winner at about 8:30 p.m. EST, several hours before the polls had closed in about half the states, including New York. Reagan had won a majority in the Electoral College, based on network projections.

In 1980, they made a similarly early projection of Reagan as the winner. Jimmy Carter also raised the ire of West Coast Democrats by conceding several hours before the polls closed in the western states.

Democrats complained that these early calls of the presidential races may have cost them several House seats and seats in state legislatures because many Democratic voters concluded that there was no point in voting.

The networks' agreement was the result of negotiations with Reps. Al Swift (D-Wash.), chairman of the House Task Force on Elections, and William M. Thomas (R-Calif.), its ranking minority member. In a joint news conference to announce the agreement, Swift and Thomas emphasized that they preferred a voluntary agreement.

"We have always believed that a voluntary approach by the networks toward participating in a solution was far preferable to any effort by Congress to legislate what they could or could not do," the two said.

They indicated that the networks' agreement and a uniform closing law will be required to eliminate early projections of a presidential winner because, in many elections, the winner won a majority of electors before West Coast polls closed.

"With this agreement, the solution is now on the shoulders of Congress," Thomas said.

I'm confident we can provide a uniform poll-closing law," Swift added. Both agreed that chances are "a little better than 50-50."

Some congressmen expressed doubt that Congress has the constitutional power to dictate poll-closing times to states, but Thomas said their study indicates that "we have the power to do so for federal elections. The states and local jurisdictions would probably not want different closing times. There is enough sentiment for solving the problem that people will go along . . . . "

One crucial consideration is that a uniform closing time disrupts the current system as little as possible. Swift and Thomas said they think closing the polls of all states except Hawaii and Alaska at 10 p.m. EST would be least disruptive, and they said they tend to reject keeping the polls open 24 hours or opening West Coast polls on Monday night.

They said the agreement was the culmination of behind-the-scenes negotiations with heads of the networks.

ABC, which was most cautious in its projections in the 1984 election, reportedly agreed immedidately when the congressmen approached them in early December. NBC and then CBS agreed later.

The network presidents -- Roone Arledge of ABC, Lawrence K. Grossman of NBC and Edward M. Joyce of CBS -- said they had long favored a uniform closing time in presidential elections.