Whenever Americans get into a dispute with the federal government these days, their first instinct often is to write their congressman for help.

The 435 representatives and 100 senators so frequently are called in as intermediaries for constituents that acting as an ombudsman has become a major part of serving in Congress.

The congressmen can't always win the case for their constituents. But they can make the agency sit up and get moving. And that's all most letter-writers are asking for-to get the bureaucracy off its duff.

Now, a new form of congressional Ombudsmanship is emerging involving the tax code, and it's threatening to go farther than ever before - by virtually nullifying whatever law the constituent is upset about.

If the Internal Revenue Service issues an unpoplar ruling, all a company, or individual need do is raise a sufficient ruckus. Then Congress steps in and takes over the issue, and often the taxpayers get off scot-free.

The new congressional meddling already has tied up three controversial IRS ruling, and appears about to block another one-this time the emotion-laden IRS attempt to enforce denial of tax exemptions to so-called segregation academies.

The three earlier issues include: Fringe benefits. The IRS ruled last year that fringe benefits-such as discounts on plane tickets for airline stewardesses-were taxable as income under the Internal Revenue Code, and began dunning taxpayers for them.

But the move raised such an coutcry that Congress went crashing into the scene. The lawmakers voted three times to block the fringe benefit regulations. They have yet to resolve the issue.

Independent contractors. The IRS issued tough rules year that would limit the categories of workers who could classify themselves as self-employed and thus avoid having to pay withholding taxes.

But West Coast timber-cutters and others wanting to escape the withholding tax bind took their appeal to Congress. Result: The lawmakers rolled back the IRS action, after months of delay.

Ips. The advent of the credit card has uncovered a dining room dilemma: Waiters and waitresses apparently aren't reporting all the tips they receive. So, the IRS issued rulings requiring restaurants to report charge-card tips.

But the restaurant industry went to Congress, and the lawmakers finally rolled back the ruling, at the same time prohibiting IRS from moving any further. The whole process took several months.

Now, the lawmakers appear ready to enter still another area-IRS's new efforts to crack down on so-called segregation academies, private schools, which were set up as havens for white pupils fleeing biracial public schools.

The IRS issued proposed regulations on the issue, and-after a strident outcry from private church-related schools-is revising its rulings some. But the changes may never make it Congress already has set hearings on the issue.

Admittedly, Congress isn't entirely the villain in this fracas. By any rasonable standard, most analysts consider the IRS's rulings on the independent contractor issue a bit too harsh, and in need of some revision.

And agency officials concede, in retrospect, that their criteria for judging private shools did not take account of special cases. If the Amish operate a church school, it's not likely to include many blacks.

But tax experts are troubled about two points:

First, whether it's generally good practice for Congress to be getting into individual rulings, as it has been in recent years.

No one denies the lawmakers ought to be riding herd on the way IRS is run. But experts say challenging individual rulings this way handicaps IRS-and leads wouldbe tax-avoiders to believe they have an easy escape.

Second, the fact that congressional entry into a tax foray virtually freezes IRS action on the issue for months.

It would be one thing if Congress got into the fray and then took steps to resolve it quickly. But the more frequent pattern has been for the lawmakers to delay any controversial decisions - in effect barring all enforcement.

In the case of the fringe benefits issue, for example, both court actions and IRS enforcement decisions are waiting for a congressional decision. Thousands of taxpayers are being left in a sort of limbo.

It's clear to a good many analysts here that the lawmakers and the revenuers need to get together a bit on some of the more controversial IRS rulings in hopes of averting a flap when the agency issues its proposals.

But if Congress insists on stepping in as ombudsman, there are those who wish the lawmakers would come armed with a proposed solution as well as a stay order. The tax code has enough gray areas already without Congress causing more.