Tuesday night may be the time we find out how much of FDR there really is in Ronald Reagan.

The televised speech Reagan will give on tax reform could be a signal event in his presidency and a landmark in the long-term future of national politics. But that will be true only if he has the boldness -- and the vision -- that distinguished his first political hero, Franklin D. Roosevelt.

Ever since the tax-reform issue surfaced last year, it has been clear that it offers the Republican Party a way to identify itself with the interests of millions of working-class and middle-class families. More than any other issue, it allows Republicans (if they have guts) finally to bury the historical liability of being seen as the party of big business and the fat cats.

Treasury I, the blueprint for tax reform put out by the Treasury last December, did that. It astonished veteran tax-reform advocates and left a good many Democrats shaking their heads in dismay that the Republicans had stolen their lunch.

Some of those Democrats had urged their own 1984 nominee, Walter Mondale, to campaign on a plan not quite so bold as Treasury I. But Mondale demurred, choosing the course of caution. When they saw Treasury I, those Democrats feared Reagan would harness the public anger at the unfairness and the staggering complexity of the existing tax system, and use the issue to cement voter loyalty to a populist Republican Party.

But in the last few weeks Washington has been filled with reports that the Reagan administration is in headlong retreat from the bold principles of Treasury I. Supposedly, Treasury negotiators have swapped concessions to this and that interest group in return for their support -- or at least, their grudging acceptance -- of the remainder of the package.

As the rumors have circulated, the Democrats have come to think they may be reprieved. While Treasury has fiddled with its own proposal, congressional Democrats have put their trademark on a corporate minimum-tax proposal that will be a key ingredient of any tax reform package Congress approves.

If they can plausibly make the case that Reagan's package is loaded down with special-interest exemptions, then the final, watered-down measure Congress passes will be just another in an endless series of tax bills. Republicans will have lost the political high ground -- and their chance to use tax reform as a lever to gain majority status in America.

But Reagan still holds the decision in his hands, and in his memory. Unlike almost anyone else in 1985 politics, Reagan remembers how FDR was denounced in his time as "a traitor to his class." Rich and privileged men could not understand how the Squire of Hyde Park came to be the champion of the New Deal.

But Reagan also remembers that Roosevelt broke the bonds of his own political heritage and created a coalition that made the Democratic Party the dominant institution in this country for two generations.

Will that memory, and that instinct, guide Reagan's words and actions Tuesday night? My guess is yes. We know that the Roosevelt myth is deeply embedded in Reagan's consciousness, so deeply that he quoted FDR in accepting the Republican nomination for the presidency -- to the shock and chagrin of many of the delegates in Detroit.

Reagan's biographer, Lou Cannon, reminds us that Reagan remained a Democrat well into middle age, largely because of the hold FDR had on his imagination. Even after Reagan shifted parties, "he refused to abandon the (FDR) words and phrases which provided a shared language and a common bond with his fellow citizens."

Tuesday is the time -- and maybe the last time for this second-term president -- to seek major policy and political gain by emulating not just Roosevelt's rhetoric but his bold tactics. I think Reagan will seize the moment in the most dramatic fashion possible, by divorcing his party from the corporate sponsorship that has been its sustenance -- and its curse -- since the days of Mark Hanna. I think he will astonish and outrage some of his own longtime friends and backers, but chill the blood of the Democratic opposition and forever change the way that millions of people think about the Republican Party.

I think he will talk about the loopholes wealthy individuals and corporations use to avoid paying their taxes and promise most taxpayers lower rates and simpler forms. And then I think Reagan will say:

"Some of the examples of unfairness come close to me personally. As most of you remember, I was for many years the host of the General Electric Theater on television, and a spokesman for GE around the country. I have great respect for the people at GE and the contribution that company has made to our country. But I think it is wrong that the average worker on the GE assembly line pays a quarter of his or her earnings in income taxes, while that profitable company pays nothing. And that's one thing our plan will change."

Am I dreaming? Maybe, but watch what happens if he does. You'll see the landscape of American politics change. And if you listen closely, you may hear the ghost of FDR chuckle.