President Reagan is expected to name Chris Arcos, a veteran U.S. Information Agency official now on loan to the State Department, to head a new "public diplomacy" campaign aimed at building support for continued U.S. military assistance to the Nicaraguan contras.

Arcos, 40, is in charge of public diplomacy for Latin America at the State Department. He will report to White House communications director Thomas C. Griscom and work closely with former representative Tom Loeffler (R-Tex.), who was named earlier this week to head the administration's legislative lobbying effort for aid to the contras fighting the government of Nicaragua.

Loeffler and Arcos will be the point men for what Reagan admininstration officials acknowledge is an uphill battle to continue U.S. assistance to the rebels opposing the leftist Sandinista government. The current appropriation of $100 million in aid for the contras expires Oct. 1; the Reagan administration is seeking $120 million for fiscal 1988.

A Day to Mark . . .

The Census Bureau is looking ahead. It has asked the nation's calendar makers to observe the 1990 census by marking April 1 that year as "U.S. Census Day." As bureau Director John G. Keane puts it, "The decennial census is one of the few governmental activities that touches every household in the nation."

The 1990 census will be the 200th head count: The first national census was taken in 1790 when George Washington was president and Thomas Jefferson, as secretary of state, was chief census taker.

Leaving Office . . .

Linda E. Carlisle, special assistant to the assistant secretary for tax policy since 1985, leaves the Treasury Department today for a return to law. She's best known on Capitol Hill for her work on the Tax Reform Act of 1986.

Stephen D. Hayes, chief spokesman for the Federal Aviation Administration, has left after 20 years of government service to become vice president of communications for the Air Transport Association.

Heroine No More . . .

Although 52 senators had signed on as supporters, Snow White lost out yesterday when the Senate Judiciary Committee decided the fairy tale heroine was too commercial to have a week set aside in her honor.

The week of July 13 marks the 50th anniversary of the premire of the Walt Disney movie, the first full-length cartoon film. Some committee members objected to a special designation because it violated rules on commercial activities. There was no immediate comment from the Seven Dwarfs.Based on staff reports and news services