WASHINGTON — The U.S. Department of Justice is facing heightened pressure to investigate whether Roger Clemens committed perjury in testifying to Congress earlier this month, following a written request sent yesterday by two senior lawmakers.

In a letter to Attorney General Mukasey, Rep. Henry Waxman, a Democrat of California, and Rep. Tom Davis, a Republican of Virginia, said the former pitching ace’s sworn testimony on February 13 denying steroid use was “directly contradicted” by Brian McNamee, a former trainer, and by a teammate, Andy Pettitte.

“We are not in a position to reach a definitive judgment as to whether Mr. Clemens lied to the committee,” Waxman and Davis wrote. They are the respective chairman and ranking member of the House Committee on Oversight and Government Reform. “Our only conclusion is that significant questions have been raised about Mr. Clemens’s truthfulness and that further investigation by the Department of Justice is warranted. We ask that you initiate such an investigation.”

Allegations of steroid use have already cast a dark shadow over Clemens’s 24-year baseball career, and a possible criminal probe would bring the added prospect of a felony conviction and prison sentence.

He has consistently denied the accusations and willingly testified before the congressional panel in part to try to clear his name. McNamee testified that he injected Clemens with performance-enhancing drugs at least 16 times beginning in 1998. The charges, some of which were corroborated by Pettitte, were initially outlined in the 409-page report on steroid use in Major League Baseball published in December by a former Maine senator, George Mitchell.

Clemens refused to discuss the referral when questioned by reporters in Florida, where he is serving as a spring training instructor for the Houston Astros. But his attorney, Rusty Hardin, has said repeatedly that he expected the Justice Department to get involved.

“Roger has known since December that if he publicly took the position he has taken, this would be the result. The good news is we are now going to be on a level playing field,” Hardin told the Associated Press yesterday. “These matters are now going to be decided in court and by the ultimate lie detector — a jury.”

A Justice Department spokesman, Paul Bresson, said officials were “reviewing the letter” and had no further comment.

The letter carries no legal force, but former federal prosecutors interviewed yesterday said it would be unusual for the department to decline an investigation request from members of Congress.

“There’s no way they’ll ignore it,” a former U.S. attorney for the District of Columbia, Roscoe Howard Jr., told The New York Sun. “If they ignored it, I’d be shocked, and my guess is you’d find some people looking for jobs.”

Howard said the Justice Department would likely refer the case to the U.S. Attorney’s Office in Washington, where it would be assigned to the FBI and one or more prosecutors.

For baseball, the larger concern may be that while Waxman and Davis asked only for a criminal probe on Clemens, it is unlikely an inquiry would stop with him.

“Federal investigations just have tentacles that a lot of times it’s just hard to anticipate. They reach in long directions,” Howard said.

“They start with this tome, this Mitchell report,” he added, “and there’s no way they’re just going to carve out a chapter or paragraph and say, ‘This is all we’re going to deal with.’ My guess is they’re going to look at the whole thing.”

The government oversight panel last month asked Justice officials to investigate whether the 2002 American League Most Valuable Player, Miguel Tejada, lied to the committee in 2005 when he said he never used performance-enhancing drugs and had no knowledge of other players using them. The referral also cited the Mitchell report, and the FBI has opened a preliminary investigation, the AP reported.

An IRS agent involved in the prosecution of Barry Bonds, Jeff Novitzky, attended the February 13 hearing in which Clemens and McNamee testified.

Federal officials may also feel pressure to probe Clemens because of its ongoing prosecution of Bonds, baseball’s all-time home run king. The government in November charged Bonds with perjury and obstruction of justice in connection with his testimony to a grand jury in which he denied knowingly taking performance-enhancing drugs.

“While it’s not a political institution, it will look at political questions,” a former federal prosecutor who is now an attorney with Goodwin Procter in New York, David Pitofsky, said of the Justice Department.

Perjury is a tricky charge to prove in court, because prosecutors must demonstrate that a defendant deliberately lied, and the potential deterrent effect of bringing a prominent case must be weighed against the chances of securing a conviction. “Frankly, the most important thing is to get it right,” a former U.S. attorney for New York’s eastern district, Zachary Carter, said. “It does not send a great message to bring a failed prosecution.”

In addition to the Justice Department letter, Waxman yesterday sent an 18-page memorandum to Democratic committee members detailing seven assertions by Clemens in his testimony “that appear to be contradicted by other evidence before the committee or implausible.”

Those assertions not only include Clemens’s denials of taking steroids or human growth hormone, but also his denial that he attended a 1998 party at the home of Jose Canseco; his testimony about receiving injections of lidocaine and B-12 vitamins; his testimony about conversations with Pettitte and McNamee about HGH, and his testimony that he was “never told” that Mitchell wanted to speak with him.

Waxman cited as contradictory evidence to Clemens’s denials of steroid and HGH use the testimony of McNamee and Pettitte, along with medical records from 1998 kept by the Toronto Blue Jays that detail treatment of an injury Clemens allegedly sustained as a result of injections to his buttocks.