The greatest challenge to tax overhaul in the Senate is being mounted not by high-priced corporate lobbyists but by people like Ruth Koach, Al Friedricks and about 160 of their officemates who want to protect their Individual Retirement Accounts (IRAs).

Koach, a receptionist at Continental Telephone Co. in Dallas, recently learned with alarm from a television news program that the tax bill would repeal IRA deductions for people like her, who have company pension plans. She went to Friedricks, the company lobbyist, who agreed to draft a protest letter to Sen. Lloyd Bentsen (D-Tex.).

Friedricks gave the letter to Koach, left for some appointments, and "when I got back I found out that 160-odd employes had signed it," Friedricks said. "That's about 80 percent of our work force."

"This is not some campaign generated by the big banks," Friedricks said. "You want to talk about real grass roots? This is it. The feeling around here, to be honest, is that the middle class is going to pay for this tax bill no matter what the politicians say, and the IRA is something that helps people in the middle."

The drive to save IRAs has emerged as the leading threat to the Finance Committee's bill, precisely because it appears to be spontaneous -- in contrast to orchestrated campaigns for capital gains, real estate, state and local sales-tax deductions and other write-offs that the bill would curtail.

The constituency for IRAs is enormous. Banks, brokerage houses and mutual funds have aggressively marketed them since 1982, when Congress made all Americans eligible for the tax-deferred accounts. Today, about 30 million people have IRAs, with total holdings of $250 billion.

Under current law, all salaried and wage-earning Americans can deduct on their tax returns up to $2,000 a year in IRA deposits, and accumulate interest tax-free throughout their working lives. Someone in the top tax bracket of 50 percent effectively pays only half-price for his or her IRA, thanks to the write-off.

Taxes are collected only when the money is withdrawn at retirement -- when the person's income generally is lower, as is his or her tax rate.

The Finance Committee's bill would repeal the annual $2,000 deduction for people covered by private pension plans, but it would allow all Americans to continue using IRAs, and to accumulate interest tax-free -- a more valuable benefit over the long term.

The change, which industry officials say would affect about 20 million households, would raise an estimated $25 billion in new tax revenue over five years, according to the Finance Committee.

It is not clear whether senators will restore all IRA deductions when the tax bill comes to the floor this week. The overall idea of the bill -- which would slash rates for businesses and people in return for curtailing many popular tax breaks -- drew unusual enthusiasm from voters across the country during the Memorial Day recess, according to a survey of senators.

Moreover, under the austere ground rules of tax overhaul, every deduction restored must be balanced by the repeal of other tax benefits. The $25 billion price tag for restoring full IRA deductions would be almost impossible to finance without tampering with the low tax rates that are the heart of the bill's appeal.

Still, senators' offices from every region reported IRA protest letters in the thousands, and aides said that almost all of them appeared to have been individually written, like Friedrick's, as opposed to form letters.

The vehemence of the grass-roots IRA lobby surprised many senators. Normally, the greatest resistance to tax revision comes from organized business groups, with individual taxpayers scarcely heard from until after they sit down to do next year's tax returns.

Such an outpouring is all but certain to affect floor action in an election year, particularly because the Senate debate is to be televised.

Because of the public response, the industry lobbies are largely sitting on the sidelines. The mutual-funds industry, which holds $40 billion in IRAs, bought radio ads in early May urging nationwide protests of the Finance Committee measure. But a spokesman said last week that the funds' chief trade association plans no further ads because grass-roots reaction has overtaken anything a mass-media campaign could generate.

The banking lobby is also out of the picture -- at least officially -- because its trade organizations fear that restoring IRA deductions could mean repealing other tax benefits important to banks. Banks hold more than half of the nation's IRA accounts, industry officials say.

That is not to say that large corporations are sitting on their hands. The brokerage firm of Merrill Lynch, which publicly supports tax overhaul, recently sent a mass mailing to its IRA customers (the firm has more than 1 million IRA accounts, the most in the country) encouraging them to write, wire or telephone their senators to protest the proposed changes. The names, addresses and phone numbers of all 100 senators were attached.

Certain mutual funds that marketed IRAs heavily in recent years plan similar mailings, according to industry officials. A spokesman said millions of IRA customers likely would stop making deposits if the Finance Committee bill passed -- a potential blow to the industry.

Several senators said last week that the fury over IRAs appeared to be dissipating, in part because people now realize that tax benefits for IRAs are not being eliminated, only curtailed. "It's not as severe as senators first thought, because the first word was: the Finance Committee abolished IRAs," said committee Chairman Bob Packwood (R-Ore.).

Several senators emphasized, in addition, that lower rates would make IRA deductions less valuable. People now in the 50 percent bracket would move to the 27 percent bracket under the Finance Committee bill. Rather than saving $1,000 in taxes for $2,000 of IRA deposits, they would save $540.

As is typical in such high-stakes fights, both sides have marshaled academic studies to support their positions. Some studies indicate that IRAs have stimulated new savings -- a key goal when IRAs were created -- and others show they have had no effect.

Some emphasize that the vast majority of IRA holders are middle-income people, while others show that the percentage of IRA holders among taxpayers at the top of the scale is almost 10 times greater than the percentage of people who hold IRAs while earning $30,000 or less in adjusted gross income.

These studies mean little to Friedricks and his coworkers. According to Friedricks, "Most people don't know much about tax reform. It's too complicated. But when it comes to IRAs, they feel like the federal legislators were reneging on a commitment to them. Even the president said it was a good thing."

Last month, however, Reagan abandoned his earlier call for expanded IRA benefits. He said he would trade a big part of the existing deduction for a Senate tax bill that reduces rates to 27 percent.

And Senate Majority Leader Robert J. Dole (R-Kan.), who earlier floated plans for an amendment restoring certain IRA deductions, last week said he would oppose such a move in hopes of moving the committee package and preserving its top rates of 27 percent for people and 33 percent for corporations.