The Republican National Committee has picked up Interior Secretary James G. Watt's tab for two controversial private Christmas parties he and his wife threw at Arlington Cemetery's historic Custis-Lee Mansion.

Committee Chairman Richard Richards sent Watt a $6,517.30 check on Nov. 2, Election Day, along with a warm letter defending Watt's use of an official National Park Service fund to pay for the two social events.

Richards wrote the beleaguered interior secretary that he hoped the financial aid from the GOP would enable him to "put all thoughts of this controversy behind you and move forward with even greater success than you have realized in the past 20 months."

Watt provoked a public uproar when he used a fund intended to "further the mission of the National Park Service" to pay for a Dec. 14 breakfast for Cabinet wives hosted by his wife, Leilani, and for a Dec. 17 cocktail party for Republican appointees and Interior Department officials.

A General Accounting Office investigation, ordered by a House committee, concluded that neither affair furthered "Park Service purposes," and that Watt should pay for them, either out of his own pocket or his official entertainment fund.

Richards, noting in his letter that he attended the cocktail party at the historic mansion, said he disagreed with the GAO and thought it was "only fair that we Republicans bear the cost of these events."

Richards said he considered the parties worthy official Park Service expenditures because he and many Republican appointees enjoyed "learning of the Park Service programs" from officials at the party.

"If I were Park Service director, I could not think of a better investment of funds to further my program than was made that evening," he wrote.

The cost of the cocktail party included $2,300 rental for a tent pitched outside the mansion overlooking Arlington National Cemetery; $2,700 for catering and $55.96 for four baskets from Pier I Imports, according to Interior documents.

Another reason the GOP decided to pay for the parties, Richards wrote, was that Watt "invited, in addition to the National Park Service personnel, many of the new Republican appointees -- people who disrupted their lives, oftentimes gave up large amounts of income and, without exception, made personal sacrifices to do what had to be done to suppport the president and yourself."

Interior paid $1,825.68 in overtime to employes who attended the party, including six park police officers and 14 National Park Service employes, according to a department report.

The Christmas parties have been a bitter point of contention for several months between Watt and Democrats on the House Interior and Appropriations committees who oversee his actions. Rep. Sidney R. Yates (D-Ill.), who chairs the Appropriations subcommittee on interior affairs, recently threatened to cut the full cost of the parties from Watt's 1983 salary unless he paid up.

Watt, meanwhile, has steadfastly maintained that it is standard Interior Department procedure to pay for such functions out of the Park Service's Cooperating Association Fund. The fund is made up of contributions from private, nonprofit groups that sell educational materials at the national parks.

In forwarding the RNC check to the Park Service yesterday, Watt wrote in a memo that the GAO opinion "can only be characterized as politically motivated and symptomatic of the poor staff and management of that organization."

Richards echoed this theme in his letter to Watt, saying: "It is truly regrettable that Mr. Bowsher Comptroller General Charles A. Bowsher saw fit to join ranks with those members of the House of Representatives and their staffs who are compelled to blindly lash out at you and the principles for which you stand in any spurious way that they can."

GAO officials could not be reached for comment.