Consider this hypothetical problem in practical politics. A new president is committed to three economic measures: 1) to increase defense spending, 2) to cut non-defense spending, and 3) to cut personal income taxes, over the next three years, by 30 percent. The question: as the president's legislative opposition, which of those three measures do you have the best chance of defeating?

Obviously, the answer must be Option 2, because that involves the taking away from some people of something they already have. Option 1 would only be possible if a pacifist-isolationist fever were abroad in the land. Option 3, of course, is unthinkable. A president-backed income tax cut is the closet thing there is to a legislative lock.

Still, some 48 hours before the Democratic House gave Ronald Reagan his most important victory since Nov. 5, 1980, the House Democratic leadership was confident it had the votes to defeat the Reagan tax plan.

The alternative Democratic tax bill had been confused by some eyewitnesses with the president's tax bill. Both bills were being changed almost to the last minute as the result of a nuclear arms race of "investment incentives." In previous administrations, investment incentives were sometimes called tax loopholes. But the Democratic bill was the one that had been written by the Democrtic Ways and Means Committee. That was the way you could spot it.

This was before the president spoke to the nation on Monday night. The president set up a television "roadblock"; he was on all networks, on every channel except the ones you have to be a safecracker to get on your dial. And the president was apparently very persuasive.

Ronald Reagan and his political advisers must have understood the state of mind of more than a few House Democrats. A number of them are feeling scared and vulnerable some 15 months before their next election -- feeling vulnerable especially to an attractive opponent with a $500,000 budget and a popular Republican president. That's what they've been hearing about. A number of those House Democrats were ready to sue for peace with the Republican White House, or simply to surrender.

By now, Ronald Reagan has been called a brilliant communicator a couple of million times. He is that. But at some point, the message, as well as the messenger, must get some credit. People respond to the Reagan message. On Monday night, he asked people to do something pretty simple: to help him to cut their taxes by contacting their congressmen. A lot of people were willing to spend a few minutes and a coule of bucks to back the president.

To observe that the congressional Democrats are in disarray is the functional equivalent of discovering that Henry VIII was not monogamous. The Sepaker of the House is visible, and therefore he is criticized. Of course, the philosophical redefinition of the Democratic Party was never in the speaker's job description before 1981. It is tough anytine to build a coalition and hold a party together fighting popular tax cuts championed by a popular president.

O'Neill, in what is fast becoming a new political ritual like baby kissing, was televised while telephoning his congratulations to the president. "No hard feelings, old pal," said the speaker to the president. But some hard feelings or sour grapes were present on the part of some Democrats. Those were the partisans who had the nerve to criticize Reagan's alleged deal making for votes on his tax bill -- less than a year after congressional Democrats were openly criticizing a president of their own party for his failure to make "deals."

Now the president has all that he wanted, and more, in the way of a tax bill. Still, even among some of his supporters there is more hope than confidence about the effectiveness of his economic recovery plan. Reaganomics reminds some people of the hot fudge sundae diet, reported in our supermarket press. You know, the ice-cream-only plan where, in only three weeks, the lady in Oklahoma lost 80 pounds, all her blemishes, and her nephew to a UFO. It would be awfully good if it worked, because not much else has.

Americans of all kinds, even including House Democrats, genuinely want the president's economic recovery plan to work. For the president's sake and his party's, it had better. Because as of the first of August, the American economy was Ronald Reagan's.