When Rep. Barbara Kennelly (D-Conn.) set out to amend the tax-revision legislation pending before the House Ways and Means Committee, she knew what she wanted to do -- increase the standard deduction for single parents -- but she didn't know who would be helped most by the amendment.

Nor did she know where she could find additional tax revenue to "pay" for her amendment, so that it would not increase the federal deficit.

But, like other members of the tax-writing congressional committees, she knew where to go: the Joint Committee on Taxation, whose phalanxes of staff aides provide technical assistance to tax writers on both sides of the Capitol.

Within two weeks, Kennelly had estimates of how much federal revenue her amendment would lose, how much the deduction for other taxpayers had to be reduced to make the package "revenue-neutral" and which income brackets and types of taxpayers would be affected. Her amendment was accepted.

"I don't see how we could have done it without them," Kennelly said. "Their analysis was very good. If they give you enough numbers, you can do anything."

They are the drones who make tax revision possible. Without the squadrons of economists, accountants and lawyers of the joint committee, President Reagan's chief domestic initiative would still be on the drawing board. If tax revision fails, it won't be because of inadequate staff work. If it succeeds, the Joint Committee on Taxation will have survived perhaps the greatest challenge in its 59-year history.

Almost every action Congress takes is made possible by anonymous aides who lay the groundwork, do the research and take care of details. That is even truer with taxes, which are so maddeningly arcane that even the most knowledgeable members of the House and Senate tax-writing committees have trouble finding their way through the Internal Revenue Code.

The nonpartisan joint committee is one of the most highly respected institutions in Congress.

Its revenue estimates -- an incredibly intricate set of numbers that only the joint committee and the Treasury Department produce -- serve as the basis for the Ways and Means Committee's decisions on tax overhaul, and committee staffers are constantly consulted on amendments. They also played a key role in producing the tax-overhaul options proposed by committee Chairman Dan Rostenkowski (D-Ill.).

Unlike other legislative committees, the joint taxation panel is its staff. The committee's pro forma congressional members are the senior members of Ways and Means and the Senate Finance Committee, but they hold no additional authority as a result of those positions, and Joint Tax has no power to enact legislation. Unlike the Joint Economic Committee, it does not meet even in an oversight capacity.

Instead, the committee staff provides advice of an economic or legal nature. Most, including chief of staff David H. Brockway, are tax lawyers, often with experience in the private sector.

Especially during a tax-writing season like this one, they often work seven-day weeks, staying until the wee hours of the morning. They also are paid more money than most other congressional employes. Brockway, for example, earns $73,600 a year, while his deputy, economist Randall D. Weiss, earns $68,700. Members of Congress earn $75,100 a year.

For years, the committee's 40 or so professionals kept a low profile. When tax bills were written in closed session, as they were until 1975, staff members could operate almost invisibly. Even now, they generally speak to the press only on the condition they not be identified or quoted in connection with the committee in any way.

Other factors also have changed since the days of Wilbur Mills, the Arkansas Democrat who ruled Ways and Means from 1958 to 1974. In the 1960s and 1970s, congressional tax writers could spend months debating tax legislation, with little public pressure and little controversy. Members debated provisions in an almost academic way, then held one up-or-down vote on the final package.

"It used to be much smaller and more intimate," said Mark McConaghy, former head of the joint tax panel who is now with the accounting firm of Price Waterhouse. "Members talked freely when they were debating a tax bill, and there was more time to do it."

Nor was there as much concern over federal revenue back in the good old days. When almost every tax measure either cut taxes or at least did not raise them, the joint-committee staff could devote fewer resources to figuring out whether every proposed amendment would increase the deficit.

Now, even though Ways and Means has returned to closed sessions, the pressure is intense and the pace -- at least for the staff -- frenetic. Every one of the 36 members has his own agenda and his own amendments to propose, and the joint taxation committee has to analyze each one of them before they can be proposed.

Brockway keeps track of those amendments in two bulging spiral notebooks, with amendments indexed by the name of the member of Congress proposing it. But estimating revenue, a complex process that involves myriad assumptions about human behavior and the functioning of the economy, takes time, and Ways and Means doesn't have much of that. As a result, revenue estimates on a particular amendment are subject to frequent revision.

For example, projections provided by the joint committee on a proposal two weeks ago that would have cut the taxes of commercial banks were so fuzzy that legislators voted for the amendment out of exasperation, some Ways and Means members said. (They reversed themselves the following week.) Members with other amendments claim, with varying amounts of supporting evidence, that the joint committee's numbers are not always correct and that their amendments often cost less than projected.

Rep. Beryl Anthony (D-Ark.), for example, contended that a compromise amendment affecting savings and loan institutions would reduce revenue by $600 million over five years; the joint committee estimate was $1.2 billion.

"Any request we make, they have to stay up all night running the numbers. You can imagine the kind of situation that creates," Anthony said.

According to analysts, the mistakes reflect a change in the entire congressional tax-writing process in the last few years. Congress now must approve massive technical corrections bills after almost every tax measure to clarify ambiguous or inaccurate portions of the legislation. The technical corrections act for the 1984 tax law, for example, is expected to run more than 300 pages.

"The problem is too much legislation too fast, with not enough opportunity for the staff to do their job an analyze it," said former joint committee staff chief Bernard M. Shapiro, who is now also with Price Waterhouse. "You didn't use to have 100-page technical corrections bills because there was time to draft. When you are under time pressure, you can't do as well."

The joint committee does not draft legislation as such. That job falls to the office of the House Legislative Counsel. But staff aides work with the counsel's office to interpret what Congress has approved. That, coupled with the power of the committee's revenue estimates and its technical advice, raises suspicions among some legislators, their aides and outside lobbyists that the committee has an ax to grind.

Brockway occasionally is accused of being biased against business, for example. Ways and Means members are sometimes irked by the staff's loyalty to Rostenkowski (on the wall behind Brockway's desk hang pictures of Rostenkowski and Senate Finance Committee Chairman Bob Packwood (R-Ore.) ).

Others say the joint committee's role is more that of a stern parent, a tax purist who discourages frivolous fiddling with the tax code.

"If you can convince Joint Tax what you're doing is right, you've won the issue," said an aide to a Ways and Means member. "It's a lot harder to get them on your side than it is to get the members" to support you.