Just as workmen are putting the finishing touches on a $3 million renovation of what used to be an indoor tennis club here for a Defense Department "think tank," the Air Force has announced that the facility and its 654 employes may be relocated to Duluth, Minn.

"Mondale," thundered Rep. Marjorie S. Holt (R-Md.) upon learning of plans to move the Electromagnetic Compatibility Analysis Center (ECAC) from her congressional district, where it has been since 1961, to Vice President Walter Mondale's home state.

"It's a blatant political move," Holt said, "part of an election year campaign by the Carter administration to hand out the goodies."

Supporting Holt's suggestion that a move might be politically motivated are Defense Department documents that insist that ECAC must be located within 50 miles of the Pentagon and other Washington area military installations if it is to carry out its critical defense mission.

Vice President Mondale was campaigning in Minnesota today. His press secretary, Albert Eisele, said Mondale was aware of Holt's charges but did not wish to get into an argument with her.

Since news of the proposal move surfaced in Duluth last week, Mondale uncharacteristically has sought to put credit elsewhere for the move that could bolster that city's sagging economy.

"Obviously the vice president is pleased," Eisele said. "But it's not something he can take credit for." The Mondale spokesman said the credit belongs to Rep. James Oberstar (D-Minn.), other members of that state's congressional delegation and "the leaders who have been working on it." s

While Obestar was anxious to get credit, there were indications that the three-term Democrat from Duluth in fact knew next to nothing about ECAC until the Air Force announced last Friday it would study moving the facility to Duluth.

And it is no secret that Mondale and other Minnesota officials have been seeking a replacement for the SAGE (Semiautomatic Ground Environment) air defense mission that is being phased out of Duluth, taking with it 2,200 jobs and a $30 million annual payroll.

Mayor John Fedo of Duluth said he talked to the vice president as his inauguration on Jan. 7 about finding a government agency to replace SAGE, and Fedo had visisted Washington twice since then seeking help "from anyone and everyone."

The loss of SAGE base, the only military installation in Minnesota, is "a life or death situation" for Duluth, a city of 100,000 whose population and economy are sinking, the mayor said.

To Holt, who is a member of the House Armed Services Committee, the transfer of ECAC would be "utterly stupid, a terrible waste of dollars desperately needed to bolster our defense."

Air Force Secretary Hans F. Mark met today with Sen. Charles McC. Mathias (R-Md) and members of Holt's staff on Capitol Hill, and emphasized that no decision has been made to move ECAC. What the Air Force announced last Friday, Mark said, was a a plan to "study the transfer" of ECAC to Duluth.

It will be at least six months before the study is completed, and Air Force spokesman said, and no move could begin until sometime after the start of the 1982 fiscal year in October 1981.

Meanwhile, workmen today were busy transforming the former Annapolis Racquet Club into a three-story office building designed to meet specifications requested by IIT Research Center, the private contractor that employs 600 of the 640 persons at ECAC.

"They just signed a 10-year lease," said Williard Triest, an owner of the building, who added "there's one helluva penalty if they don't live up to it."

Rodney W. Ord, administrative director at ECAC for IIT (which is connected to the Illinois Institute of Technology), agreed that if ECAC fails to move in his firm could be liable for up to $1.8 million in penalties. The agreement calls for a rent of about $450,000 a year, but allows ECAC to break the lease if it pays four years' rent.

Ord said he would not like "being stuck" with trying to sublease the renovated building, but is more concerned that a move from the Washington region "will impair our mission." He added: "We have been advised for years (by the Defense Department) of the importance of being near the Pentagon and other intelligence and research and defense installations," Ord said.

There have been previous proposals to move ECAC out of the area -- to Florida, Boston and Rome, N.Y. -- but each time, the proximity to Washington was cited for rejecting those moves.

Asked today if anything had changed that imperative, an Air Force spokesman at the Pentagon replied: "The functional realtionship and the degree of involvement of ECAC within the D.C. region will be a subject of the study."

"Ecac can be moved," said Air Force Col. Thomas A. Anderson, who oversees the ECAC operation. "But the mission would be impacted. It's important that we interface" with the intelligence community scattered around the Washington Area.

ECAC employes average 100 month to Washington, he said. "If we moved to Duluth, we couldn't respond as readily."

Explaining what ECAC does is not easy, the colonel conceded. Two-thirds of its employes are college graduates, many with advanced degrees in enginering, physics and computer science.

In simplest terms, they make sure the sophisticated electronic devices that protect the nation's defense system around the world and that monitor military activities of other countries are not interferred with or do not interfere with civilian activities.

"It's like making sure the guy next door doesn't put such a powerful CB unit in his rec room that it blows out his neighbors' TV sets and starts his electric garage door going up and down," offered John A. Zoellner, ECACs civilian deputy director.

A relocation impact study conducted at ECAC in 1975 found it would take on to three years for the agency recover its peak work capability after a move because 60 to 95 percent of its employes would quit rather than move. An informal poll taken this week found that average still holds, Zoellner said.

James W. Sacra, 34, an electrical engineer, said he would not move to Duluth because "it's too cold for my wife. They'd probably lose most of us." d

Peter Mager, another engineer and one of the 40 civilians who work for the Defense Department rather than for IIT, has "no desire to move. And there is no doubt the effectiveness would be impacted," he said.

Many of the employes agree with Holt that the proposed study is an election year ploy designed to get votes for the Carter-Mondale ticket in Minnesota.

"Hopefully, after the election the pressure will be off" and the plan will be just another broken campaign promise, Holt said.