John Tower faced serious questions about scaling back Pentagon spending and about his personal ties to defense contractors as the Senate Armed Services Committee opened hearings Wednesday on his nomination to run the Defense Department.

The former Texas senator could expect to be treated with special courtesy by his former colleagues in the historic Senate Caucus Room, site of the Watergate and Iran-Contra hearings.But the 63-year-old Tower, who emerged as President Bush's nominee after an exhaustive background check, was certain to be quizzed on his consulting work for defense contractors and his private life, as well as on defense policy and overall Pentagon funding.

Sen. Alan Dixon, D-Ill., a member of the Senate Armed Services Committee, said Tuesday he expects the hearings to be marked by some controversy based on the reports that have already been raised in the media.

"There's enough out there," Dixon said.

Confirmation hearings are under way for two other Bush appointees:

- ROBERT MOSBACHER, commerce secretary-designate, was warmly received Tuesday before the Senate Commerce Committee. Lawmakers told the Texas oilman he should make the government a more aggressive advocate for American businesses in the world marketplace. His nomination is expected to win easy approval from the committee next week.

- SAMUEL K. SKINNER, transportation secretary-designate, responded in writing to questions from the Senate Commerce Committee to begin hearings on his appointment. The 50-year-old Illinois transportation administrator said foreign airlines serving the United States should be forced to meet the same security requirements as U.S. carriers.

The Tower hearings are likely to focus in part on the issue of overall military spending. Former President Reagan asked in his final budget for $315.2 billion for the Pentagon in fiscal 1990, which begins Oct. 1. The request represents a 2 percent increase after inflation.

But Bush, according to testimony last week from Office of Management and Budget nominee Richard Darman, is expected to reduce Reagan's request by an unspecified amount.

If Bush fails to trim the request, Congress is likely to do so. In the past four years, Congress has cut military spending increases to levels below the rate of inflation.

"I think they're whistling Dixie if they think they'll get a 2 to 3 percent increase in spending," said Rep. Sonny Montgomery, D-Miss., a member of the House Armed Services Committee. "They'll be fortunate if they get the inflation factor."

Tower, a former chairman of the Armed Services Committee, also was certain to be quizzed about his connections to a number of defense contractors.

According to public records and associates, Tower's consulting firm of Tower and Associates has been on annual retainer with Martin-Marietta, Textron, LTV Aerospace and Defense Co., Rockwell International and British Aerospace.