Why Mr. Reagan will debate

President Reagan is going to debate Walter Mondale because he thinks he is good at it, and that he can wallop Mondale. And he wants to give Mondale a thumping that's heard round the United States. He also hopes to impress the Soviets, in order ultimately to get an arms pact that would permit slowing the costly US defense buildup.

With approximately a 15 percentage lead, the President is taking quite a risk in moving into such a confrontation - a risk he really doesn't have to take.

There is nothing personal in Mr. Reagan's hope of overwhelming Mr. Mondale in debate. Part of Reagan's charm is that he's not a hater; he doesn't hold anything against anyone. Just as his own mistakes don't stick to him, neither do criticisms or even attacks from others.

The other morning Bill Brock, who consults with the President on political matters as well as on trade, said Reagan feels the 15 percent lead isn't enough - that he'd like to ''get 25 percent'' if he can. He believes, said Brock, that by beating Mondale badly in a debate he would increase his own lead and, hence, help put together a landslide victory to aid Republicans running for Congress.

Reagan wants a reelection mandate, more than last time if possible. He's particularly desirous of picking up seats in the House even though gaining a majority is out of reach. And he would like at least to keep GOP control of the Senate.

Reagan has ambitious goals for the next four years. He would like to improve the current prosperity - increase economic growth, keep inflation down, and decrease interest rates. He sees himself as indispensable to achieving this goal. But he knows he needs congressional help. And he realizes presidential coattails don't reach down and help very much unless a president can win by an overwhelming vote.

Reagan feels he will have a much better chance of having a summit meeting next year with Soviet leader Konstantin Chernenko and negotiating the arms pact Reagan seeks - an arms reduction if possible - if Soviet leaders see him as a strong leader who could gain Senate ratification of an agreement. Thus, as Reagan sees it, the bigger his victory the better these prospects.

More than anything else, in this campaign Reagan is looking beyond his own term. He very much wants to put together a Republican majority that will keep his counterrevolution going for years.

Reagan sees a debate with Mondale as a device by which he can build up the Republican Party - in Congress, in all public offices, and for far into the future.

Mr. Brock said the President also seeks to debate because he is thinking of cutting back somewhat on defense spending.

Some of Reagan's knife would be used, as it has been, to try to get rid of Pentagon waste. But Brock hinted broadly that Reagan would also consider the possibility of reducing the rate of the US defense buildup. Brock said this might well be one of the places Reagan would look to find spending cuts that would help reduce the budget deficit.

Reagan's desire for defense trims are linked to his basic hope that he can sit down with the Soviets and negotiate a pact to at least slow the arms race.

All of which brings us back to the debates, where Reagan, rightly or wrongly, thinks he can win so decisively that he will be able to put together a truly impressive victory in November - one that would impress even the Soviet leaders.