WASHINGTON (Reuters) - The perjury trial of former ace pitcher Roger Clemens over alleged steroid use is set to resume on Monday with a witness expected to detail why congressional lawmakers wanted to question him.

Clemens, 49, is being tried for a second time on federal charges of lying to Congress in 2008 about whether he used performance-enhancing drugs.

Clemens' trial in U.S. District Court began last week but recessed on Tuesday so Judge Reggie Walton could teach a judicial course. The trial is expected to last about six weeks.

Clemens, who won the Cy Young Award a record seven times as his league's best pitcher, is accused of lying to the House Committee on Oversight and Government Reform in 2008 about whether he used anabolic steroids and human growth hormone. The panel was investigating drug use in Major League Baseball.

The trial is expected to pick where it left off, with the first witness, Phil Barnett, the House Oversight committee's chief of staff, resuming his testimony.

Barnett told jurors Clemens had been asked to testify because he was likely the most prominent player named in a report prepared by former Senator George Mitchell.

The committee wanted to be sure it was accurate, he said.

In his opening statement, prosecutor Steven Durham said Clemens had covered up his steroid use. Proof included needles and swabs that tests have shown contained the elite pitcher's DNA and performance-enhancing drugs, he said.

Defense attorney Rusty Hardin struck back by saying Clemens had never tested positive for steroids during his 24-year career and had relied on hard work for his success.

Hardin also said Clemens' former trainer Brian McNamee, who is expected to testify that the baseball star used performance-enhancing drugs, was a liar.

Clemens faces one count of obstruction of Congress, three counts of making a false statement and two counts of perjury.

If convicted, he faces a maximum prison term of 30 years, although under federal sentencing guidelines he would most likely get 15 to 21 months.

Clemens first went on trial last July. Walton declared a mistrial because prosecutors showed jurors a video clip that included material the judge had banned from the case unless it was raised by Clemens' defense team.

Clemens played for Boston, Toronto, Houston and the New York Yankees in a career that ran from 1984 to 2007.

(Reporting By Ian Simpson; Editing by Eric Walsh)