The three major television networks beat the House to the punch last week in declaring President Reagan a lame duck. A day before the House acted by an eight-vote margin to halt further aid to the Nicaraguan contras, the networks cast their ballots by refusing Reagan free time to sum up the case for helping his beloved "freedom fighters."

The contras are fighting communism in Nicaragua, but the setback inflicted on them and Reagan by the networks was a triumph of capitalism. Faced with the choice of raking in millions of dollars from prime-time entertainment or performing the dubious public service of once more inflicting Reagan's denunciations of the Marxist government of Nicaragua on an unresponsive audience, the networks took the easy way out. Their decision was a clear signal that Reagan has lost his tremble factor with the networks.

Until last Tuesday, no one had confused the networks with a pack of tigers in their dealings with the White House. True, they had twice turned down requests for daytime television speeches, but even Reagan should realize that soap operas are sacred. Until last Tuesday, CBS and NBC had routinely honored 37 White House requests for Reagan speeches in prime time, and ABC also televised all but one of these.

Ironically, the networks rejected Reagan's speech on grounds that it contained no news. In fact, his last-minute concession to let Congress have a final say on lethal aid -- a tactic that won a few votes but not enough to pass the package -- made the speech newsier than most of its 37 prime-time predecessors. What the networks were trying to tell the White House is that Reagan will be an ex-president sooner than he thinks.

The contra-aid vote in the House also reflected this reality. As Sen. John S. McCain III (R-Ariz.), a contra supporter, put it: "Ronald Reagan is leaving next January, while {House Speaker} Jim Wright is going to be around for many years."

But is Reagan really a "lame duck" in the classic political sense? On the day that the House voted down contra aid, the Senate unanimously approved Judge Anthony M. Kennedy as Reagan's nominee to the Supreme Court. Kennedy is a fine man and respected jurist, but his decisions mark him as the kind of law-and-order conservative that Reagan always said he wanted to put on the high court. Kennedy's demeanor and political skills make him far more likely to succeed than the iconoclastic Robert H. Bork in becoming a conservative consensus-builder who carries on the Reagan judicial legacy.

The dual message of the two votes last Wednesday was that Reagan can carry a Democratic Congress with him, but only when he carefully chooses his causes and candidates. This condition did not begin in the eighth year of the Reagan presidency. Most of Reagan's legislative triumphs came during his first 18 months in office, when the normal political honeymoon was extended by admiration for the president's heroic behavior after he was wounded in an assassination attempt. He lost what little legislative clout he retained when Democrats took control of the Senate in 1986.

White House chief of staff Howard H. Baker Jr. argues that the idea of a lame-duck president is a "political anachronism" in a nuclear age and contends that fundamental national-security decisions cannot be delayed while the nation awaits the next president. He has a point. Last December, Reagan and Soviet leader Mikhail Gorbachev signed the first treaty that eliminated a class of nuclear weapons, and the Senate seems certain to ratify this popular accord. The two leaders also have an opportunity to make significant progress toward a more important strategic nuclear arms treaty at a planned Moscow summit this spring.

What Reagan must do, after the contra vote, is be as realistic in dealing with Congress as he has become in negotiating with the Soviets. The unanimous vote for Judge Kennedy suggests what Reagan can accomplish when he pays attention to political reality. Cooperation, not confrontation, is the key to being a successful lame duck.

Reaganism of the Week: Speaking to the National Prayer Breakfast last Thursday, the president said: "I have long been unable to understand the atheist in this world of so much beauty. And I've had an unholy desire to invite some atheists to a dinner and then serve the most fabulous gourmet dinner that has ever been concocted and, after dinner, ask them if they believe there was a cook."