In a city where politicians regularly put the arm on political action committees for campaign cash, Sen. James Abdnor (R-S.D.) last week came up with an aggressive new tactic to use on Washington's PAC community.

The senator's campaign committee, Friends of Jim Abdnor, dispatched paid messengers armed with letters from Sen. Strom Thurmond (R-S.C.), on stationery with letterheads reading "The President Pro Tempore United States Senate," to the offices of corporate and trade association PACs.

The messengers were instructed to wait in each PAC official's office until the director read the letter, which said, in part: "President Reagan has stated that our No. 1 political priority for 1986 is keeping the Republican Senate majority . . . for that reason, President Reagan is attending a fund-raiser for Jim.

"I am sure that the president will be disappointed to hear that your PAC has not signed on to attend the president's tribute to Jim. Please fill out the attached card and send it back with the messenger who delivered this letter."

The card gave the PAC director three choices:

*Give "the maximum contribution of $5,000."

*Buy one or more tickets at $1,500 each.

*Say "No, our PAC does not wish to support the Salute to Jim Abdnor featuring President Reagan."

Thomas Mason, Abdnor's administrative assistant, said the use of paid messengers assigned to wait for an immediate answer "worked. We got some of the answers we wanted."

A number of PAC directors, however, were angered and one ripped up the letter and threw it in a wastebasket, Mason said.

Mason said all the receipts have not been counted, but he estimated that the fund-raiser, held Tuesday night, raised about $225,000.

"It was a good inside-the-Beltway event," he said.