The doors of Capitol Hill's newest marble monument, the Hart Senate Office Building, were officially opened yesterday, but 25 of the Senate's most senior members have already said no thanks, they won't take their offices to Hart.

As senator after senator has declined an invitation to move from the Russell or Dirksen office buildings, it becomes more likely each day that some senators will be ordered to pack their office supplies and unwillingly move to Hart.

A total of 14 senators -- including Senate Majority Leader Howard Baker and Minority Leader Robert C. Byrd -- have agreed to move their offices to the Hart Building, named after the late senator from Michigan, Philip A. Hart.

But since the Senate Rules Committee has decreed that 50 senators will have their offices in the Hart Building, it now seems likely that most of them will be the newest, most junior senators, who do not have the seniority to stake a claim to one of the 36 offices that will remain in Russell or 14 in Dirksen.

The cold shoulders that many of the Senate's senior members are now turning to the Hart Building are only the latest chapter in what has become a long-running congressional sideshow. Controversy has virtually been the Hart Building's middle name.

It has been reviled as a latter-day Taj Mahal, a building viewed by its critics as too extravagant for half of the United States Senate when millions of their constituents are standing in unemployment lines. Whatever the asssessment of the building it is, with a price tag of $137.7 million, already one of the most expensive federal structures ever built and the final cost may still rise.

"The prime reason why many aren't moving is that it's too palatial, too opulent," said Sen. William Proxmire (D-Wis.), one of the building's prime critics and one who once gave the Senate his monthly "Golden Fleece" award for approving the Hart construction. "They don't want to give someone a chance to say, 'This guy's had six years of lush living, so let's kick him out.'"

Spokesmen for some of the senior senators say, however, that the proximity of their offices in the Russell and Dirksen buildings to both their committee hearing rooms and the Capitol, and the quality of their existing offices more than offset the benefit of the extra office floor space they would gain with a move to the Hart Building.

"We like it here," said Rex Buffington, spokesman for the Senate's most senior member, Sen. John C. Stennis (D-Miss.), who now occupies what once was the vice president's Capitol Hill office in the Russell Building. "This office is really a nice suite. We have three [nonworking] fireplaces . . . and . . . wood in them from Mississippi."

Sen. Charles McC. Mathias (R-Md.), the chairman of the Rules Committee, is also keeping his office in Russell. "He's kind of an old-shoe type who goes for the marble fireplaces," explained spokesman Jack Eddinger. "They're kind of classical [with] vintage woodwork."

What the senators moving to the Hart Building will find is a large white marble and glass building with a nine-story atrium topped by skylights, 16-foot-high office ceilings, bronze elevator doors and a rooftop tennis court for their relaxation hours.

"This gives us a chance to double our room space," said Gregg Takayama, spokesman for Sen. Daniel K. Inouye (D-Hawaii). "We'll be gaining a view of the city for the first time." That, he said, will be a marked improvement over the current view for Inouye staffers: rubbish and occasional dead birds and animals that pile up in the window wells outside their subterranean offices in Russell.

For his part, Architect of the Capitol George M. White, who oversaw the long construction of the Hart Building, said he is not surprised that most senior senators are staying put. He also is unswerving in his view of how history will judge the Hart Building.

"I think it's a great building," the architect said. "It's a lot of value for the money."