Charitable organizations pleaded with the House Ways and Means Committee yesterday to retain a tax code provision that lets taxpayers who do not itemize their deductions write off contributions to charity.

They had a sympathetic audience: More than half the members of the House support the provision.

Charities fare much better under President Reagan's tax-simplification plan than they did under the earlier Treasury Department version, which would have limited the deductibility of charitable contributions for all taxpayers. The current plan would repeal the deduction for non-itemizers and include a portion of gifts of property in the tax base used in calculating a new minimum tax designed to keep well-off taxpayers from paying little or no taxes.

But representatives of universities, museums, foundations and philanthropic groups said yesterday that reducing tax rates and ending the deduction for non-itemizers would cost them $11 billion -- almost as much as the projected loss from the Treasury plan -- and was unfair to low-income taxpayers.

"Because the president's bill repeals this deduction, it fails the fairness test by discriminating against the vast majority of Americans: those who do not itemize their tax returns," said Jack Moskowitz, senior vice president of United Way of America.

The 1981 tax law extended the charitable-contribution deduction on a phased-in basis to the 63 percent of taxpayers who use the standard deduction, rather than itemizing deductions. That deduction will expire at the end of 1986 unless the law is changed. Reagan's tax plan would end it a year early, on the grounds that it is difficult to administer and that the standard deduction is intended to cover charitable contributions.

A bill to make the deduction for non-itemizers permanent has gained 231 cosponsors in the House, more than half that body's membership, and most members at the hearing appeared sympathetic to retaining the deduction.

The charitable deduction "is not just another special-interest deduction," said Rep. Harold E. Ford (D-Tenn.)." . . . A gift to a soup kitchen is simply not the same as the purchase of a three-martini lunch."

Economic research has shown, however, that the issue is not as simple as it appears. Charitable contributions, like all deductions, are worth more to high-bracket taxpayers than they are to lower-income persons. Even under the Reagan plan, one dollar of contributions saves 35 cents in taxes for an upper-income taxpayer and 15 cents for those in the lowest bracket.

For that reason, lower tax rates alone would reduce contributions to charities by lessening their value as tax breaks. Almost half the reduction in contributions that the Reagan plan would cause would come from the effect of lower rates, not from ending the non-itemizer provision.