You can't go wrong by telling people what they are already thinking, or asking them for something they have already given you. That's what Ronald Reagan did at his last news conference.

The president knows from his pollster, Richard Wirthlin, pretty much how the American people feel about him, so whenever he could last Wednesday night, Reagan did what the technicians call "reinforcing."

In his opening statement, for instance, he spoke of his wish that economic recovery "could be easier and faster." But he immediately swung into a line he knows strikes a chord in the country:

"It's tough, slow work, and it's going to require enormous effort and patience from every one of us . . . . "

Wirthlin has told him that he has what every president dreams of and seldom achieves, "a pool of patience."

The end of the sentence was also provided by Wirthlin: " . . . to correct the problems we inherited."

That was easy, too. Wirthlin's surveys show that a majority of Americans think that present economic woes are "the result of years and years of the wrong course, and that the Congress shares the responsibility."

Reagan cannot, of course, hide or disguise what is happening 16 months into his administration. The recession, high unemployment figures and business failures are not classified material. But apparently Reagan can get away with saying that it's because he did not get what he wanted.

When he signed the two principal instruments of his economic revolution, the budget reconciliation and the tax bill of 1981, no one heard him saying that he had gotten half a loaf. In fact, the signings were triumphal occasions at which he hailed "the single most important achievement" of his young administration, achieved through a marvelous bipartisan coalition in Congress.

But hear him in the Oval Office last week: " . . . I can say back to them, 'All right, then why don't you just give us what we've asked for?' "

Congress thought it had.

But from Wirthlin's whispers, Reagan knew he did not have to be tethered to the facts.

Wirthlin checked around in May and found out that of 1,502 voters who were asked how much of Reagan's economic program has been passed, 43 percent said they thought about half; 34 percent thought it was "less than half."

Why should he not splash about in his "pool of patience" until the word gets out?

Reagan's greatest luck, of course, is that the people have not yet made the connection between him and his policies. They do not blame him for what is happening, even, it seems, on the unemployment lines.

He represented to them, when they voted for him, promise and hope. The prospect of lower taxes, less regulation and reduced government spending sounded like the Promised Land to them. To give up on him so soon would be a way of giving up on themselves, Wirthlin thinks.

As if the people were not giving the president enough, House Democrats gave him more last week. They decided to accept, as is, a Senate tax reform bill that would raise $98.5 billion over a three-year period and go right to conference with it--even though the Constitution says that revenue-raising bills should originate in the House.

The Democrats only noticed that the bill is the work of a Republican senator, Robert J. Dole (Kan.). They did not take in the wonder of it, which is that it is fair. It takes raw courage to raise taxes in an election year, especially for Republicans, and the right wing is outraged, charging that "the Republican Party is in danger of making a U-turn back to its familiar role of tax collector for Democratic spending programs."

But the Democrats, it appears, do not have the advantage of Wirthlin's counsels. If they had, they might not have been so fast off the mark.

The issue of fairness is their strongest campaign weapon. Out there, voters have thought from the first that Reagan is soft on the rich.

Wirthlin calls unfairness the president's "most severe perceptual liability." The Democrats had a chance to make the new taxes even fairer. But all they saw was a Republican tax bill 90 days before the fall election, and a chance for a little campaign gloating.

Only James M. Shannon (D-Mass.), in the Democratic caucus of the Ways and Means Committee, voted against sending the measure to conference. He argued that the House could make a good bill better and share responsibility for a responsible action. His colleagues would have none of it.

"We ought to stop trying to be so damn cute on every issue," Shannon says. "We want it both ways, we say we want lower deficits, but it isn't our tax bill--don't blame us."

The Democrats' campaign slogan is: "It isn't fair--it's Republican." Now they have chopped it in half.

You can call the tax bill Republican, but you can't call it unfair.

As Wirthlin keeps telling him, Reagan's luck is phenomenal.