It's been called a latter-day Taj Mahal, an extravagant affront to the American taxpayer, and a palace "that would make a Persian prince green with envy." Some time this fall, it will also be the official home of half the United States Senate.

The penthouse dining room overlooking the Capitol has been eliminated and the gymnasium scrapped under pressure from the voters. But for the 50 senators about to move into the new Philip A. Hart Memorial Office Building, Capitol Hill's latest marble monument, it will be their home on the Hill whether they want it or not.

The Hart Building has been 10 years in the planning and construction, and at the official current cost of $137.7 million -- a figure that may grow considerably when the final bill is tallied -- it is already one of the most expensive federal structures ever built.

Each senator's office in the new building has a private washroom and 16-foot office ceilings "to inspire them to the dignity of the office people elected them to," in the words of Hart's architect, John Carl Warnecke. There is a nine-story atrium with large skylights.

Three-inch-thick, gray-streaked white marble slabs, twice the normal thickness for large buildings, cover much of the facade. Marble also covers the rooftop penthouses for mechanical equipment. Inside, there are bronze elevator doors, while outside, bronze "handhole covers," providing access to wiring, are embedded in the sidewalk. The idea is to avoid the unsightly rust stains that cheaper cast-iron covers can cause.

Underneath the building, the loading dock has concrete pillars sheathed in stainless steel.

The Capitol subway has been extended to speed the members to the Senate floor, and important congressional hearings can be held in a large two-story room specially equipped for massive television coverage.

When they want to relax, the senators will be able to use a rooftop tennis court.

The net result, despite the design changes, is still an enormously elaborate building. Conceived in an era of big government spending with relatively little concern for its cost, it is being finished and judged in an era of budget-cutting and acute consciousness of government waste.

Its creation is the product of the longstanding and perennial desire of senators and their staffs for more work space. Since a second Senate office building, the Dirksen, was finished in 1958, the Senate's staff has grown from 2,500 workers to 7,000. By the standards of the General Services Administration, Dirksen and the older Russell Building provide less than half the recommended floor space for each Senate office worker.

As originally conceived, the Hart project was going to create a mirror-image duplicate of the Dirksen Building, at a cost of $47.9 million. But as the years passed, it was transformed into a building that, while attached to Dirksen, has its own architectural character as well as a controversial construction history that rivals that of the problem-plagued Rayburn House Office Building built in the 1960s.

Hart evolved, and its cost has soared 187 percent, through the peculiarly Washington chemistry of politics and the prerogatives of power. To a large degree, Hart was the brainchild of George M. White, 61, the architect of the Capitol, a man with a fiefdom of 2,300 employes and an annual budget of $90 million in addition, some critics would say, to a penchant for visions of grandeur. But White could do nothing without the approval of his boss, the Senate, which at times imposed its changing will on the Hart Building's features, as new members replaced old ones, and at other points gave its silent blessing to the changes White was implementing.

Whatever the changes, one result was certain: The cost kept going up. Inflation, rampant in the construction industry over the last decade, raised the costs still further.

White says he and Warnecke, whose firm will be paid more than $6 million as associate architect for the Hart project, made most of the major design decisions for the building, often improving the type of materials used as the construction progressed.

"My feeling is that the taxpayers get their money out of these expenditures," the blunt-spoken White said. "Over the long run, you're better off with good quality. There's nothing gold-plated over there. It ought to glisten a bit; we're a great nation. . . . We believe a building on Capitol Hill ought to last as long as the nation wants to maintain it."

The Senate occasionally has debated the merits of the building. At one point, it argued at length whether to finish the two-thirds completed superstructure, while in 1978 the House, in a rare incursion into the Senate's traditional domain, temporarily rejected a $54-million construction appropriation for Hart. Just last month, the Senate approved and then killed a $736,400 expenditure to finish the proposed Hart gymnasium, which for now will remain a large unused room. But most of the daily construction supervision has been left to White.

"I think we've pretty much trusted the architect," says Sen. Robert T. Stafford (R-Vt.), chairman of the Senate Office Building Commission. One Hart critic, Sen. William Proxmire (D-Wis.), says that is exactly the problem with Hart: "The Senate has just let the architect of the Capitol go wild."

White now concedes, "Obviously there are instances of mismanagement. I'm not trying to minimize inadequacies or other errors."

But White, a slight, outspoken man with a liking for narrow bow ties, quickly adds, "When someone says, 'How much is a building going to cost?,' everyone tends to overlook if you bought a quart of milk or loaf of bread in 1971 you're not going to pay the same today. There's been massive inflation in the construction industry."

Nonetheless, he says it is a "fair statement" to conclude that he and his staff were not aggressive enough in their oversight of the construction work.

The Hart Building is named after the late Democratic Sen. Philip A. Hart of Michigan, a wry and understanding man who doubtless would have been amused by the rancor evoked by the building. The building's critics -- nearly half the Senate, based on several votes over the years -- firmly believe it is nothing short of a boondoggle, the latest example of Washington's excesses.

Proxmire once gave the Senate his monthly "Golden Fleece" award for approving the Hart construction and last month led the fight to delete funding for the gymnasium. He calls Hart "a ridiculous waste of money."

Another critic, Sen. John H. Chafee (R-R.I.), said the Senate's message on Hart to the taxpayers is simple: "Be frugal, work hard, get the country back to its basic principles. Our actions are: Take care of ourselves in the most luxurious manner possible." Chafee's 1979 effort to halt construction failed on a 49-to-46 vote.

The latest fight over Hart developed in the last several months. Architect White found earlier this year that "through good luck" he had $4.2 million left in construction money that could be used to pay for items that previously had been trimmed when the Senate placed a $137.7-million spending ceiling on the project.

At the behest of Sen. J. Bennett Johnston (D-La.), one of Hart's staunchest supporters, the Senate Office Building Commission approved the additional items, including the gymnasium, on a 4-to-1 vote, with Stafford dissenting.

Proxmire thought the $4.2 million should be sent back to the Treasury. But when he proposed doing just that, the Senate Appropriations Committee left no doubt where it stood, rejecting the idea 19 to 3.

"The gym was the most vulnerable," Proxmire said of the priorities for the extra spending that Johnston had drafted. So Proxmire forced the gym issue onto the Senate floor, singling it out as an excess that even the paunchier members of the Senate did not need.

But he lost, 50 to 48.

It wasn't long, however, before senators started hearing from outraged constituents and their mailbags were filled with vitriol. How dare the Senate vote itself a gymnasium?

The Senate, seeing the error of its ways in an election year, recanted, voting 98 to 0 to mothball the gym.

Hart has its Senate supporters as well. The role of chief apologist has fallen in recent years to Johnston, who was chairman of the Senate Office Building Commission when the Democrats controlled the Senate.

"Notwithstanding the cost overruns," he said, "by and large the building is going to be a triumph. To me, it's marvelous. It's going to be quite functional. We've got a building that's worthy of the Senate and the United States.

"I'm unashamed and unrepentant," Johnston declared. "I'm damned proud of it."

White said he expects the first Senate workers will move into Hart in the first part of November, 10 years after the architect was first asked by a Senate Appropriations subcommittee how much it would cost to build a new Senate office building. With just two weeks to come up with an answer before the subcommittee met, White said, he had no time for any detailed study.

"So we've been saddled with $48 million as the original cost estimate," he said with a tone of exasperation. "It's inappropriate. It's a deception."

By early 1974, with the expanded scope and quality of the project, the cost had jumped to $68.8 million and by the end of that year to $85.1 million. It went to $137.7 million three years ago.

Construction-related problems plagued the project almost from the start. By all accounts, some of the anchor bolts installed in the building's foundation by the George Hyman Construction Co. were misaligned. What remains in dispute is how many of the bolts were misplaced and what problems that caused Baltimore Contractors Inc., which followed Hyman on the project and used the bolts to attach the building's superstructure.

William Magruder, Hyman's marketing manager, said that through "a construction screw-up" less than 5 percent of the 500 bolts in the concrete foundation were wrongly placed.

Victor J. Frenkil, the flamboyant board chairman of Baltimore Contractors, said 90 percent of the anchor bolts were askew, causing his firm a 237-day delay by the time the problem was rectified.

For Frenkil, the controversial construction executive whose hobby is folding dollar bills into the shape of the initials of friends and acquaintances and then giving them away, it is just the latest dispute with White's office.

For more than a decade Frenkil has pressed his $4.5-million claim that White's office failed to properly inform Baltimore Contractors about the condition of the soil on the site where Frenkil's firm built the House underground parking garages, causing undue excavation problems. The case is still before the U.S. Court of Claims.

In all, Frenkil has filed more than $13.5 million worth of claims against White's office on the Hart project, contending that mistakes by the architect's office caused innumerable delays. None has been settled, but any that are would once again boost the cost of Hart.

"I would say the way the Capitol architect's treating us," Frenkil said, "the only way we'll get recovery is if the courts give us a fair shake . . . . "

White and others think Frenkil's claims are exaggerated. The Capitol architect has conceded only that Frenkil's firm may be entitled to some money for the anchor-bolt problem, nothing else.

Whatever the outcome of Frenkil's claims, other costs for Hart or related to it invariably will boost the final bill. White, for example, has requested an additional $450,000 to install new energy conservation devices in Hart and $405,000 for electronic security devices.

But for those who think the escalating costs of Hart are seemingly endless, White has a history lesson.

He enthusiastically showed a reporter a 1909 New York Times article about the construction of the Russell Senate Office Building, built at a cost of $5 million, that described it as "a veritable palace . . . a building that a thousand men would feel lonesome in."

"They were screaming about $5 million then," he said. "That building was controversial. There's never been a building up here that hasn't been."