Banks and savings and loans have been offering their depositors an expanding range of financial services recently and now they're lobbying Congress for a measure to make it easier for customers to cheat on their income taxes.

Of course the embarrassing phrase "income tax evasion" never appears in the fliers from bank and savings and loan lobbyists that are barraging depositors and Capitol Hill.

The financial institutions say they merely want their customers to "help repeal a bad law"--the one that will require banks to withhold 10 percent of interest paid on most accounts starting July 1.

"Warning! 10 percent of the money you earn in interest is going to DISAPPEAR," screamed one photocopied letter that arrived in last week's mail.

Even by the standards of political propaganda, this is an outrageously misleading statement. Nobody's taxes will be raised by the withholding of interest income. The tax rate itself doesn't increase, nor does the amount of interest income subject to taxation.

Under the interest withholding law, taxes on interest income will be deducted automatically in the same way that taxes on salaries are deducted from your paycheck.

The only depositors whose money is going to "disappear" are folks who've been cheating on their income taxes.

That's a lot of people. Estimates are that 15 percent of taxable interest goes unreported. One out of every $8 in taxable interest income already "disappears" before the tax collectors can find it.

Income tax evasion used to be something akin to a mortal sin, if not obscene than at least unpatriotic.

Now, apparently, tax cheating has become socially acceptable, close enough to running a yellow light that banks can subtly tell customers that part of their interest income will "disappear" if taxes are withheld from it.

Assuming that the interest income tax that is withheld is interest income tax that otherwise wouldn't be due, the financial institutions have concocted an elaborate series of phony arguments against withholding interest income. Listen to this one from the U.S. League of Savings Institutions:

Witholding interest "will erode the nation's store of investment capital" and "send shock waves through the investment community," League Chairman Leonard Slane claims in a speech he's scheduled to give today at a convention in Miami Beach.

He says the tax withheld on a $10,000 savings certificate earning 12 percent a year, credited monthly, would be $126.76 the first year, $141.23 the second year, and $157.34 the third year, for a total of $425.33. "It wouldn't be long before the account shrank by more than $1,000," argues Shane.

If I were basking in Miami Beach today with Mr. Shane on his tax-deductible business trip, I'd ask him how much income tax would be due on the account in three years if the tax were not withheld? Would you believe $425.33?

Shane claims taxpayers "are scrupulously careful about paying all taxes due on interest and dividend income." That's not what you'll hear from Sen. Robert Dole (R-Kan.), one of the most adamant backers of withholding interest income.

Dole is not exactly a Robin Hood, rob-the-rich type when it comes to taxes. He's a leader of the campaign to reduce the holding period on capital gains from a year to six months to encourage savings and investment.

Sponsors of the withholding measure contend that a huge amount of taxes goes uncollected because investors don't report their interest income. They estimate the government will pick up $10.5 billion in additional revenues in the next three years and $25 billion by 1988.

Besides the tax cheats who are pocketing that $25 billion--and forcing the rest of us to pay their share of running the government--the real losers from interest income withholding will be the banks. Which may explain why they are so concerned that their depositors' money is going to "disappear."

No question about it, the banks are going to have to pay the cost of collecting that interest tax for Uncle Sam. To compensate the financial institutions, the government is going to let them keep the money they withhold for 30 days before they turn it over to the IRS. The banks will get a month's free use of the tax money.

That's not enough, according to a study done for the banks by Peat, Marwick, Mitchell & Co. The Peat, Marwick report, which is being used by the financial industry in its lobbying campaign, contends the cost of withholding interest income will be 10 to 100 times as much the banks will earn from the 30-day float.

It's difficult to argue with the banks' bookkeeper's estimates, but on the face of it, the Peat, Marwick numbers seem a little steep. The claim of 100 times one month's interest that's being bandied about by the bankers is equivalent to more than eight years' float.

Whatever the cost, the banks and savings associations aren't the only businesses that help the government collect taxes.

Retailers collect sales taxes, and many of them have to invest in fancy cash registers that can be programmed to distinguish between taxable and nontaxable items. Oil companies collect gas taxes for both the state and federal governments. New car dealers collect license plate fees. Real estate agents collect transfer taxes. Distillers and tobacco companies stamp every pack and bottle to prove the appropriate taxes have been paid.

Every employer in the nation has been withholding income taxes from workers' checks since 1942. In 40 years, it's doubtful that many companies--let alone banks--ever went broke because of the onerous cost of handling employe withholding.

Whether they like to do it or not, all kinds of businesses help make tax collection relatively painless and surprisingly efficient. That's why the United States has one of the most respected, best-observed tax systems in the world. There's no reason why the banks and savings institutions shouldn't do their part instead of lobbying to make tax evasion easier.