President Reagan comes naturally to his fear and loathing of "unjust taxes," which he denounced last week while announcing his far-reaching plan to overhaul the nation's tax system.

Reagan discovered the downside of the federal tax system when he returned to his movie-acting career in 1946 after four years of making wartime training films in Hollywood. Reagan, at 35, was struggling to regain his status as a budding star in a competitive industry torn by technological change, costly strikes and the looming menace of television.

He found the road back difficult, but his reentry into civilian life was eased by a $3,500-a-week contract at Warner Brothers, which his agent had negotiated on the strength of Reagan's fine 1942 film, "Kings Row." That salary, which increased over the years, brought Reagan an annual income of nearly $250,000 in the late 1940s, about the equivalent of $1 million today.

This newfound prosperity came when the top tax rate was 90 percent, tax averaging was prohibited and tax shelters -- now an accepted scandal in American life -- were mostly reserved for corporations. Reagan, who came from humble origins and considered himself a liberal Democrat, found much of his new wealth went directly to the Internal Revenue Service. Some friends from the movie days believe this discovery of progressive taxation moved Reagan toward his conversion to Republicanism.

In any case, there is no doubt that Reagan felt himself a victim of the tax code. He felt so strongly about it that he even advocated what he called a "human depreciation allowance," patterned after the tax break allowed the oil industry.

The issue never caught on, but Reagan emerged from this formative period with a lifelong enmity for taxation. "He really despises taxes," an intimate from his movie days told me in 1981. "He considers taxes an evil."

As Reagan's income status and ideology changed, he acquired a political outlook that undergirded his personal views. On the stump for General Electric in the 1950s, Reagan became an apostle of lower corporate income taxes. Later, as governor of California, he campaigned for an initiative to cap government spending that he believed would brake state and local taxation. The initiative failed, but it proved the forerunner to Proposition 13, the anti-tax initiative overwhelmingly approved by California voters a few years later.

Reagan expressed his true feelings last week when he unveiled his tax plan with a call to "radically change the structure of a tax system that still treats our earnings as the personal property of the Internal Revenue Service."

His views are more radical than his plan. Reagan has long opposed corporate income taxes on grounds that corporations simply pass on the taxes to individuals through higher prices. White House chief of staff Donald T. Regan says this "great tax theory" of Reagan's "still holds in the longer term, but it's not practical to achieve that."

Instead of campaigning for abolition of corporate taxes, therefore, Reagan has been persuaded by Regan and Treasury Secretary James A. Baker III to crusade for a proposal that would raise corporate taxes but distribute them more fairly. One irony of the plan is that it would increase the tax burden on Reagan's former employer, General Electric, which currently pays no taxes. Another is that it would eliminate income averaging, a provision Reagan once considered essential to fairness. But Reagan's belief in a fair tax system is genuine and rooted in his experiences.

Reagan's father sold shoes. Reagan sold refrigerators and good will for GE and the intangible commodities of optimism and faith in the achievements of free enterprise. The GE slogan, "Progress is Our Most Important Product," could well be the battle cry of the Reagan presidency.

All salesmen know that they must believe in the product they are selling before they can sell it to others. Reagan, the premier political salesman of our time, is always best when he is on the road or before a camera plugging something close to his heart. The tax plan qualifies. It will take a lot of selling, but Reagan has found the central issue of his second term.

Reaganism of the Week: Speaking at Colonial Williamsburg Thursday about "demeaning" IRS practices, Reagan said, "Wasn't there a line a couple of hundred years ago about being safe in your books and your records?"