he government today began to reveal the details of its conspiracy case against Washington defense lawyer William A. Borders Jr., asserting that it would prove that Borders, acting as a middleman for a federal judge, took $25,000 in bribe money and later showed up at an Arlington motel to collect another $125,000.

Borders and U.S. District Judge Alcee L. Hastings of Miami are accused of seeking the $150,000 payoff from two brothers convicted of racketeering in Hastings' court in December 1980. In return for the cash, the prosecution said it would prove, Hastings ordered the refund of $845,000 that he had earlier required the brothers, Frank and Thomas Romano, to forfeit after their convictions.

In his opening statement today, Justice Department attorney Robert I. Richter said secretly taped evidence would show that Borders asked: "You got it with you?" when he arrived at the Marriott Twin Bridges motel last Oct. 9, allegedly to pick up the second installment of the bribe money from an FBI undercover agent who was posing as Frank Romano.

"Inside," the undercover agent, Paul Rico, replied, Richter told the jury. The motel room had been wired by law enforcement officials to make video and audio tapes of the meeting, Richter told the jury, but Borders did not enter the room as expected.

"Get it. I want to take a ride," Richter said Borders told Rico at the doorway. Tapes of this conversation were made on a recorder that Rico had concealed under his clothing. Rico and Borders then got into Borders' car in the motel parking lot, Richter said. The $125,000, in $100 bills, was in a travel bag on a seat in the car, Richter said.

At that point, other FBI agents observing the undercover operation, realizing that their plan had been upset and fearful that they could not protect Rico, turned on sirens and arrested Borders, Richter said.

Hastings, who will be tried separately in Miami, and Borders have denied all charges of wrongdoing. Defense lawyers for Borders delayed making an opening statement until after prosecutors have finished presentation of their evidence.

Richter, prosecuting the case along with Justice Department lawyer Reid Weingarten, said the government's evidence would show that on Sept. 19, Borders took $25,000 in "front money" from Rico at the Miami International Airport. The money was in an envelope hidden in the folds of a newspaper, according to prosecution testimony today from FBI special agent William J. Murphy Jr. of Miami, who supervised the investigation.

Murphy also testified that when Borders left the airport with the money, he acted "very secretive and worried, as if he was being followed."

During cross-examination by defense lawyer John A. Shorter Jr., Murphy said he had no idea what happened to the $25,000 that Borders allegedly took with him and testified that Borders was not followed by the FBI when he left the airport.

Undercover agent Rico, who was called out of retirement to pose as Frank Romano, was introduced to Borders by an informant who had told the FBI about the alleged conspiracy between Borders and Hastings, the government has said. Richter said that the evidence would show that when the two first met, Borders tried unsuccessfully to persuade Rico, who was wearing a body recorder, to write on paper rather than talk about amounts of alleged bribes.

Richter also told the jury that other wiretap evidence in the case includes a conversation between Hastings and Borders. Richter contends that the government's evidence will show that the conversation, while couched in what he described as "extremely guarded" language, relates directly to the alleged bribe scheme, which hinged on Hastings' decision to reverse the forfeiture order against the Romanos.

"I drafted all those letters for him and everything is okay," Hastings said on the tapes, Richter told the jury.

"I talked to him and he wrote some things down," Borders replied on the tape, Richter told the jury.

"I understand," Hasting replied, according to Richter.

"Then I was supposed to go back and get some more things," Richter told the jury Borders said.

"All right, I understand," Richter said Hastings replied.

Rico, the undercover agent, is expected to take the witness stand Wednesday.