The effort to overhaul the federal tax code was jerked out of gridlock during three productive days of drafting by the House Ways and Means Committee last weekend, leading committee Chairman Dan Rostenkowski (D-Ill.) to predict that his members could produce a bill before Thanksgiving.

But even within the committee, serious doubts remain that something as complex and sensitive as a new tax system is attainable in so short a time.

"If a journey of a thousand miles begins with a single step, we've taken a step and a half," said Rep. Thomas J. Downey (D-N.Y.).

Three questions remain unanswered after the committee's most successful period of tax-writing so far:

*Does Rostenkowski, after intense negotiations with members, hold a working majority that will support most of his proposals to curtail deductions in return for lowering rates?

*Do members have the political nerve to eliminate tax advantages for favored interests rather than compromise on such proposals in ways that add complexity to the tax code?

*Can the committee find enough additional tax revenue to "pay" for its compromises without raising rates so high above what President Reagan wants that the administration will abandon ship?

During the weekend sessions, Ways and Means members who had been reluctant to support many of Rostenkowski's tax-overhaul proposals became more pliable, possibly as a result of one-on-one meetings with the chairman last week.

While he apparently made no direct commitments, Rostenkowski raised the possibility of retaining the popular deduction for state and local taxes and the tax-free status of employer-provided health insurance. In return, members would help him with the rest of the bill.

The strategy appeared to pay off. Rostenkowski gained 22 votes on the 36-member committee, for example, for a package limiting the use of tax-exempt bonds. Among his supporters were all three committee members from New York, even though the bonds are widely used in that state.

An amendment by Rep. Harold E. Ford (D-Tenn.) to exempt hospitals and universities from the bond limits failed on a 17-to-18 vote, with three Republicans voting on Rostenkowski's side against it. One of those Republicans was Rep. Raymond J. McGrath -- of New York.

If those deals were made, they have a darker side that may bode ill for tax revision in the long run: Analysts point out that Rostenkowski seems to have gained his majority not by turning members into tax reformers but by letting them keep their favorite loopholes.

How long that bargain will last as other parochially important provisions come up is unclear. The most controversial proposals, affecting such powerful interests as labor, the defense industry, manufacturing and high-tech firms, have not yet come before the panel.

"There is a working majority, but that's like saying, how far have we traveled on a land-mine road?" said Rep. Charles B. Rangel (D-N.Y.). "This thing can explode at any time."

Rostenkowski has achieved clear victory in few cases so far, accepting compromises in such areas as the bonds, small business, low-income housing, timber and agriculture.

The panel has even voted to create new tax breaks, approving, for example, a 20 percent tax credit for manufacturing companies with less than $25 million in sales.

The committee's compromises would add complexity to an already complex system. Big banks would be taxed differently from small banks, big timber producers differently from small producers and unemployment compensation differently from workers' compensation. The tax code is messy now, and the committee so far has not moved toward making it cleaner.

"There is less and less tax reform in [the bill] every day that goes by," said Rep. Willis D. Gradison (R-Ohio). "It will bear a distant family relationship to the president's plan, but no one would think of it as an ideological twin."

Some would contend Reagan's plan was not pure reform either. But it did eliminate or curtail enough tax breaks to accommodate a top tax rate of 35 percent (the current top rate is 50 percent) without bringing in less revenue than the current system.

Rostenkowski -- who began his work with the same rate structure of 15 percent, 25 percent and 35 percent -- is having to look hard for revenue to replace the tax breaks that have been reinserted into the legislation.

The compromises have put the panel $15.9 billion over five years below the "baseline" set in the chairman's package.

And the potential deals the chairman has raised in sessions with members would cost even more. Restoring the full deduction for state and local taxes, for example, would cost $65 billion over the five years. Not taxing any of the health-insurance premiums paid by employers for their workers would cost another $3 billion, and countless other in-between solutions can be expected to increase the shortfall.

Committee members say personal tax rates will have to rise to cover those compromises. But the task of adding higher rates is trickier than it sounds. The president's insistence on a top rate of 35 percent or less is the highest political barrier, but there are economic difficulties as well.

The problem, as explained by government tax experts, is that there are not enough rich taxpayers to pay for restoration of many expensive deductions. Less than 1 percent of all taxpayers, 828,000, had taxable income of more than $100,000 in 1983.

As a result, adding a fourth bracket of 40 percent on top of the proposed system would raise only $45 billion over five years even if it kicked in at the relatively low income level of $100,000 for a married couple.

Studies by the congressional Joint Committee on Taxation for members of the Ways and Means panel have found that raising all three proposed tax rates by 1 1/2 points, to 16.5 percent, 26.5 percent and 36.5 percent, would increase revenue by the $65 billion needed to retain the deduction for state and local taxes. But such a change would constitute a sizeable tax increase for many taxpayers in the lower brackets.

Congressional tax experts say revenue could be raised by lowering the income levels at which the rates apply, thus kicking more taxpayers up to higher brackets.

Or merely raising the top rate to 40 percent from 35 percent would bring in enough money to cover restoration of the state-and-local tax deduction.

But that device would considerably alter the distribution of the income-tax burden and is not considered a likely option.

Some members have proposed revenue-raising measures to "pay" for their amendments, but they mostly show how tough it will be to find more money: The broadest have been in the area of tax compliance -- tightening the way the Internal Revenue Service enforces the tax code, rather than closing loopholes.