As part of the new tax cut plan he disclosed this week, President Reagan has made a proposal likely to be highly popular on Capitol Hill: to wipe out estate and gift taxes for all but the wealthiest people in the country.

Members of Congress are under constant siege by farmers and owners of small businesses who argue vociferously that their heirs will have to sell out in order to pay estate taxes. The heated arguments were not diminished by cuts in the two taxes beginning in 1977, nor by the fact that family farms also now get several other special breaks. The Reagan proposal may thus help win him votes on taxes.

As a result of the recent cuts, last year only 2.8 percent of all estates owed a tax, a figure that would fall to a minuscule 0.4 percent under the Reagan plan. The proposed changes would also mean that the vast majority of families would no longer need the services of estate planning lawyers or accountants.

In 1980 the government collected $6.3 billion from the estate tax and $216 million from the gift tax. The proposal would drop that sharply as it is phased in by 1985, when the changes would be fully effective.

Reagan would make three major changes:

A surviving spouse could inherit an estate of any size without paying a tax. At present, one-half an estate or $250,000, whichever is greater, can be left to a spouse tax-free.

No estate tax would be levied until the value of an estate, after all debts were paid, passed $600,000. This cutoff at present is $175,625.

The amount that can be given by one individual to another each year without incurring a gift tax liability would be raised from $3,000, a level set more that 40 years ago, to $10,000. A husband and wife could each give a child $10,000, or a total of $20,000, and owe no tax.

Tax experts generally applauded the proposal to allow transfer of an entire estate to a spouse without tax on the grounds that a couple should be regarded as an economic unit representing a single generation. Its financial status should not be altered by a tax, they argue, that arises from the death of one spuse, who perhaps owned more of the couple's assets than the other. The true purpose of the tax is to put a levy on intergenerational transfers, not from spouse to spouse.

But while exempting transfers to a spouse has long been advocated by such experts, increasing the general exemption has not. Joseph Pechman pf the Brookings Institution was critcial of that portion of the proposal, saying, "The increase to $600,000 goes far beyond mere indexing of the exemption [adjusting it for inflation] and it involves a heavy revenue loss." e

Reagan's changes would not greatly benefit the heirs of estates worth million of dollars. The actual change would be in the form of an increase in a tax credit rather than excluding a larger portion of an estate from tax, except where a spouse is the heir. The use of an increased tax credit has the effect of providing the same dollar relief to all estates regardless of size.

At present, an estate with a taxable value of $1 million owes a tax of $298,800. The proposal would cut that by $145,800 to $153,00.

But an estate worth $5 million would get only the same amount of tax relief. Instead of a tax bill of $2,503,800, it would owe $2,358,000, a cut of exactly the same $145,800.