A group of economic evangelists has captured President Reagan's allegiance. These are the "supply-side" advocates, whose chief guru is Republican Rep. Jack Kemp. Though there have been conflicting reports about Reagan's conversion to the Kemp-Roth tax-cut plan, sources close to Reagan have assured my associate Indy Badhwar that the new president is in fact a born-again supply-sider.

The Reagan-Kemp ecnomic connection began about two years ago, after the congressman spoke at an International Longshoremen's Union convention. Kemp's address was reported in Human Events, a conservative publication that used to be Reagan's political bible. Reagan penned a letter to Kemp inviting him to California.

Their lunchtime conversation lasted three hours. Kemp recalls: "We discussed taxes, spending, growth, gold and the monetary system. I was impressed with his grasp of these subjects. But most of all I came away convinced that Reagan was a supply-sider."

Once the Reagan campaign got under way, Kemp flew out to California with charts and statistics to give the Republican candidate more information to explain the complexities of supply-side economics as opposed to the theories of John Maynard Keynes, the darling of the liberals.

What exactly was the supply-side theory that Kemp was advancing? He offers this explanation: "People work for themselves, not to pay taxes, When you want more of something you subsidize it. When you want less of it, you tax it. But under our crazy economic system, we have been taxing investments and savings while subsidizing non-work and welfare."

Kemp advoates incentives to people as the best means of restoring the nation's economic health -- rather than "pump-priming" or government aid that the liberals funnel to consumers whose purchases are then supposed to stimulate the economy.

This is the way it has been since World War II. Budget deficits are deliberately created during economic downturns and surpluses during booms. Our economic policy, the supply-side advocates argue, has been a constant choice between the lesser of two evils: higher inflation with lower unemployment or lower inflation with higher unemployment.

These "stop-and-go" tactics, as Kemp explains it, have resulted in a condition that was supposed to be impossible in Keynesian theory -- simultaneous high inflation and unemployment.

According to the supply-siders, the Keynesians favor steep income taxes, not as a way of maximizing revenues but to create consumer demand by "redistributing" income through the tax system. But the supply-siders argue this simply hasn't worked.

High taxes, they maintain, have prevented the risk-taking that is at the heart of the free enterprise system, slowed down real industrial growth, caused budget deficits and created inflation that has pushed wage earners into higher tax brackets with no gain in real income.

The supply-siders believe that the first step needed to reverse this trend is to start cutting tax rates across the board. They argue that such a move might increase tax revenues by stimulating more investment and simultaneously lessening the incentive to cheat on taxes or sock money into unproductive tax shelters.

In support of their theory, the supply-siders point to President John Kennedy's tax-cut proposals. Enacted after his death, the tax cuts led to a boom in the American economy that made possible the balanced budget in 1965 and the Great Society programs of Lyndon Johnson -- or so say the supply-siders.

President Reagan's top economic advisers are trying to persuade him that his election represented a stake through the heart of Keynesian economics and a public mandate for dramatic action in the nation's economy.

A private memo from his economic team states: "If President Reagan and the Republicans can achieve full employment without inflation we will be rewarded at the polls; if not, we will be pitched out as decisively as we were swept in."

Whether "Kempian" economic theory is ultimately sounder than "Keynesian" remains to be seen. The public, as usual, can only wait and see if the economic theorists know what they're talking about. Wait and see -- and cross their fingers.