When it comes to taxes, Americans often are a fickle bunch.

Take President Carter's new tax-revision package, for example.

Carter drew raves when he campaigned on a "tax reform" plank in last year's presidential race. Of all the issues he raised, his call for an overhaul of the tax system seemed genuinely to spark a voter response. Pundits quickly proclaimed that the "newcomer" candidate actually may have discovered the issue for the 1970s.

Now, almost 12 months later, the Carter tax package finally is about to be unveiled, and its prospects, if anything, appear to be uncertain.

It's not that the proposal isn't a sweeping one. While the outlines that have emerged so far indicate it won't be quite the dramatic turnabout that Carter promised, it's still likely to be the most comprehensive proposed by any President.

The question is, once the President has unveiled his new tax package, will anybody want it?

Rep. Al Ullman (D-Ore.), chairman of the Hoiuse Ways and Means Committee, already has indicated he's cool to the proposal, and is predicting Congress will dilute one of its key elements - eliminating special tax treatment of capital gains.

The Senate Finance Committee is expected to water down the proposals even further - and possibly add some new "loopholes" of its own.

Most important, lawmakers are reporting there doesn't seem to be much enthusiasm among the voters. "If there ever really was a so-called groundswell for tax reform last year, it seems to have disappeared," one key legislator mused recently.

In a way, the apathy isn't too surprising. When the congressional tax writing committees take up "reform" legislation from time to time, the hearing rooms are brimming - but not from crowds of voters. The overflow comes mainly from expensively dressed corporate lobbyists. The only ordinary Joes are a few tourists who happen by.

"There's just no constituency for tax reform," said a Way and Means member who has been pushing for an overhaul for the past several years. "People always say they're for it, but they never turn out."

The explanations are complex. For one thing, while most taxpayers are for "tax reform" in abstract, they seem to balk when you talk about how to achieve it.

The theory behind most "tax reform" schemes is, first, to close off existing deductions and tax preferences and then use the new-found revenues to finance big tax cuts for everyone. The computer print-outs show tax rates could be slashed substantially - and make the tax structure fairer as well.

But no matter what the arithmetic shows, the fact is that most voters just don't want to give up their deductions - no matter what the politicians promise will happen after that.

It's something akin to the stir a decade ago when planners figured out how to save housing space in land-short communities. The idea was to build homes adjacently, in cluster, and then let the owners "share" a large park-like backyard. On paper, the plan would have given everyone more land to use than the old postage stamp-sized lots. But nobody wanted it. Everyone wanted to hang on to his own plot of ground.

Then, too, despite their seeming enthusiasm for "tax reform" as a concept, voters seem automatically to balk at anything that smacks of soaking the rich.

It's the old Horatio Alger ethic. While most taxpayers seem to believe that wealthy persons shouldn't escape payment of taxes, there's a strong underlying sentiment that once someone has made it, the government shouldn't take it all away. Maybe it's a little bit of the American dream. If one of us strikes it rich, we don't want Uncle Sam cutting into it.

Finally, the outlook for the Carter package may have been dampened by the administration's own wavering. Despite warnings from House leaders to send the measure up early if they want it pushed through next year. White House officials have let it slip - from the original Sept. 1 target date to Sept. 23, Oct. 3 and now, at the earliest, Oct. 17. There's real concern in some quarters it may not be unveiled in time to begin any hearings this session at all.

Although Carter drew some of his biggest campaign cheers promising voters a bold overhaul of the entire tax system, the administration's tax writers have progressively whittled down their proposals to what they believe Congress will accept with least resistance. Unless the President decides to toughen their recommendations, while the package is a massive one, it's far from prescribing any bold new approach.

That may indeed make for more comfortable congressional politics. But what's bothering some "reform" advocates is that it also may have shorn the President of one of his biggest political chips in the 1976 campaign - his ability to cast himself as the force for thorough-going "reform." By compromising they fret, he may have lost some of that edge.

To be sure, the administration may be able to stir up new enthusiasm for its package between now and next January. The President is planning a TV appearance to unveil the proposals and a speech making tour later. But for now, there's a legitimate question in wheter, even before it's unvelled, the Carter tax program is not an idea whose time has come and gone.