How important to their party's national success -- in 1986 and 1988 -- will be the ability of congressional Democrats to unite on the upcoming fight for tax fairness and tax simplification?

In the words of one respected Democratic liberal who has held leadership roles in presidential campaigns since 1968, "This will be the acid test as to whether there exists a national party. Unless we can be the guys who'll take tax preferences away from rich 'freeloaders' in order to give tax relief to hard-working middle-income people, then we will be nothing more than the party of air bags and acid rain."

Now more than three years after Sen. Bill Bradley (D-N.J.) introduced his fair tax idea to a mostly skeptical political city, that idea has become a hot issue of political self-interest to a lot of people including President Reagan, House Ways and Means Chairman Dan Rostenkowski (D-Ill.), Republicans and Democrats.

Ronald Reagan did not carry 93 of 100 states in two national campaigns by being unable to spot and grab a hot issue. The president, we are told, was offered three alternative initiatives with which to recapture the political offensive: an anti-crime crusade, a campaign for a balanced budget constitutional amendment, or a variation of the fair tax plan.

Crime is both very real and quite personal. But domestic defense, like national defense, has the political disadvantage of requiring more than tough talk -- it means spending real money.

The president is shrewd enough to know that his advocacy of a balanced budget amendment, after submitting budgets totaling more than $600 billion out of balance, could have been derided as the functional equivalent of Bonnie and Clyde's blaming the absence of a tough federal gun control law for their bank-robbing spree. The fair tax was easily the pick of that litter, and the president, as all can see, is vigorously back on the political offensive.

That the tax fairness issue was still available in the summer of 1985 to the Republican president is one legacy of the Mondale campaign, which rejected the Bradley-Gephardt idea in favor of advocating a tax increase. In controlling their enthusiasm for the president's new tax plan, Republican members of Congress were the model of restraint. Apparently these GOP legislators fail to detect the promised political realignment that would be theirs for cutting tax preferences for business and the rich. On taxes, House Republicans look slightly less united now than they were on the recent budget fight, where their disarray made House Democrats, by contrast, look as disciplined as the pope's Swiss guard.

Led by Bradley and Rostenkowski, congressional Democrats were a lot nicer to the Reagan plan than were many in the GOP. But issuing a statement is the easy part. In Rostenkowski's judgment, his Ways and Means Committee colleagues will have to decide sometime before the end of December whether or not they want to be viewed as "protectors of privilege." The chairman hastens to insist that, regardless of how that might sound, "I'm no reformer." But Rostenkowski the Regular is already on record: "The price of rate cuts and fairness is rooting out preferences now enjoyed by some powerful interests."

Last February in discussing the possibility for tax reform before a California audience, Rostenkowski explained one reason it just might happen was because "Jimmy Baker (Treasury secretary) might like to write a little history." Later, in Washington, Sen. Bradley, when asked about the 1985 congressional fate and future of his tax plan, replied, "Danny's the key," that if the Ways and Means chairman were so determined, it just might happen.

While recognizing and emphasizing the urgent need for bipartisan agreement by his committee on a reform bill, Rostenkowski refuses to discuss the inevitable partisan clashes that lie ahead. That will be when the guys on one side seek political advantage by paying for the lowering of personal tax rates of the middle class through the elimination of preferences such as those for capital gains or the deductibility of state and local taxes.

Those fights, when they do take place, could very well reveal whether a national Democratic Party does exist and their outcome could hinge on whether in the fall of 1985 Dan Rostenkowski of Chicago decides, and proves able, "to write a little history" of his own.