Christmas is over, but last-minute shopping is just reaching the desperation point for many big spenders. They are trying frantically to beat the Internal Revenue Service by obtaining a 1981 tax shelter by midnight Thursday.

"Last year, we had two guys drop by in evening clothes on Dec. 31 to write checks on an art tax shelter," recalls Charles Walsh, an unconventional Georgetown investment counselor, who assiduously courts clients desperate for a tax break by the end of the year.

"It's an annual phenomenon," Walsh adds, "There are some very sophisticated people who buy this thing at the last minute."

Against the advice of the IRS and more traditional investment advisers, hundreds of upper-income Washington-area residents are besieging accountants in search of tax shelters. The last-minute tax breaks offered range from Walsh's highly controversial government securities "straddle" that claims to provide $30,000 in 1981 tax benefits for a $5,000 investment to a heavily advertised real estate shelter in which the developer's representative concedes he can offer little more than $3,000 in closing costs as a 1981 deduction.

Even the high-powered Walsh planned to close down his government securities shelter by the end of last week. The reamining investors will have to settle for "the worst," according to Walsh, a purchase-and-leaseback arrangement for lithograph plates from an internationally exhibited Spanish artist.

"We sell it as a quick fix," Walsh declares. The lithographic plates stored under a desk in his office are "probably the last game in town." The artist will be standing by in Washington this week to sign the plates, Walsh says.

The trim, bearded accountant wears a pinstriped suit with a monogrammed shirt and gold cuff links, although he usually favors suspenders to complement his outfit. A University of Maryland graduate, he occasionally affects an English accent. His Charles Walsh and Partners Inc. letterhead lists branch offices in London and Nassau.

Walsh sips liqueur while explaining how he answered a woman who wanted to invest $50,000 in his lithographic shelter, if he would supply a painting of a bowl of fruits instead of the abstract Spanish artist's work. "I said, 'Look lady, we're not artists!'"

Unlike most investment counselors, Walsh says, "we sell very little tax helter until after Thanksgiving."

However, Leslie J. Silverstone, a Washington vice president of the Dean Witter Reynolds Inc. brokerage firm, comments, "Once you pass the first three or four months of the year, you've lost most of the best deals." The best tax shelters include oil and gas ventures that quickly sell out every year after raising $50 million nationwide, notes Silverstone. "I am very wary of programs that are reflective of what's available at the last minute," he states.

Nationwide, the IRS is investigating 248,828 cases "involving what we feel are abusive tax shelters," says IRS spokesman Larry Batdorf. Movies and real estate are the two most investigated shelters in the mid-Atlantic region containing Washington area, according to IRS statistics.

Tax-reform legislation enacted in recent years limits deductions to the amount of money an investor actually has "at risk" in a transaction, Batdorf points out. "Many people who invest in a tax shelter are looking to generate a phoney loss," he says. "I think surely people should take a very hard look at someone who offers if you invest a dollar now, you will get a $6 loss imediately," the IRS spokesman observes. "While form an accounting standpoint you can compute" a loss well beyond the immediate investments, Batdorf says, "we would look at the economic reality of the situation."

"The response to that," says Walsh with characteristic confidence, "is that I own a house in Georgetown, a Bentley and an office with a fireplace." And he has a metal desk with a linoleum top.