Former U.S. representative Tom McMillen was arrested on Capitol Hill over the weekend after a friend told police that McMillen had pushed her down the stairs during an argument, according to the accounts of authorities and sources.

However, prosecutors said they decided against proceeding with the case when the woman later said she did not want to press charges against the Maryland Democrat. After being held for about 12 hours, McMillen, 45, was released.

McMillen's arrest, on a charge of simple assault, came about 2:45 a.m. Saturday after police were called to the 1100 block of South Carolina Avenue SE, where they found a 37-year-old woman "bleeding and bruised," according to a police spokesman.

A D.C. police spokesman, Sgt. Joe Gentile, said the woman, whom the arrest report identifies as Judy Niemyer, of Capitol Hill, said that McMillen assaulted her during an argument. A law enforcement source said the woman said she was pushed down stairs.

McMillen could not be reached for comment. A former basketball star at the University of Maryland, McMillen was a Rhodes Scholar before playing for several professional teams, including the Washington Bullets. He represented Maryland's old 4th District in Congress for three terms before being defeated for reelection in 1992 in a redrawn district.

Since then he has been involved in several business ventures. In 1993 President Clinton named him co-chairman of the President's Council on Physical Fitness and Sports.

In an interview with a reporter yesterday, Niemyer denied that she had been assaulted by McMillen, whom she called "the most gentle person in the world."

Niemyer, an emergency room physician who said she had been the former congressman's girlfriend for five years, said that she had become unnerved early Saturday when she slipped and cut her leg while carrying a heavy crystal vase up some stairs in the house where McMillen was staying.

She said McMillen was asleep early Saturday when she fell.

Niemyer said she cut her leg and foot on the vase and was bleeding profusely. She said she takes a lot of the analgesic Advil, which she said "makes you bleed more."

In addition, she said, she believed she had fractured her elbow.

"So," she said, "I'm screaming up the steps, Tom, I cut myself.' " When interviewed yesterday, she appeared to have small cuts on her leg and a bandage on her foot.

A police source said that a neighbor called police.

When police arrived, Niemyer said, "I'm standing at the front door crying. They thought that Tom had done something to me, but I'm telling you he was sleeping."

She said that she tried to explain but that police would not listen. "I tried from the moment that they got him to ameliorate the situation, but they kept at it," she said.

McMillen was in custody until his case came before a hearing commissioner in D.C. Superior Court on Saturday. At that time, the U.S. attorney's office said it would not proceed.

Assistant U.S. Attorney Terence J. Keeney, spokesman for the prosecutor's office, said the decision not to prosecute was made after Niemyer met with prosecutors at their request and told them that she did not wish to go forward.

A law enforcement source said the woman denied that she had been assaulted.

Keeney said McMillen got no special treatment.

"When we look to the facts and evidence here, our inability to prove the case cut against prosecution," Keeney said.

Prosecutors created a special unit last year to handle domestic violence complaints and occasionally have pursued cases without the cooperation of alleged victims. However, Keeney said, there was other evidence in those cases. Staff writers Mohamad Bazzi, Sari Horwitz, Amy Klein and Bill Miller contributed to this report. CAPTION: TOM McMILLEN (1990 photo)