Roger Clemens’s defense team took the offensive Tuesday, telling jurors in the District’s federal court that the government’s perjury prosecution of the star pitcher had been built on “half truths” and relied too heavily on a witness whose motives cannot be trusted.

From the moment he began addressing jurors in an hour-long opening statement, Clemens’s lead attorney, Rusty Hardin, cast the trial as a choice: Would they believe the former hurler, described by his lawyer as a hard-working, self-made man, or his chief accuser, a former strength coach with a checkered past?

The trainer, Brian McNamee, a longtime friend of “The Rocket,” has claimed he injected Clemens with performance-enhancing drugs on multiple occasions in 1998, 2000 and 2001. His allegations were included in a lengthy report commissioned by Major League Baseball, and McNamee reiterated those statements during a 2008 hearing before a congressional committee.

At the same hearing, Clemens dramatically denied ever having taken steroids or human growth hormone (HGH). The pitcher’s testimony sparked a federal investigation, and a grand jury in 2010 indicted Clemens on charges he lied to Congress when he denied having taken the substances.

On Tuesday, Hardin said federal investigators trusted the wrong man. The Houston-based lawyer told jurors that they cannot believe McNamee because he has a history of not telling the truth; in 2001, for example, Hardin said, the strength coach lied to investigators in an unrelated criminal investigation.

Hardin also called the trainer a publicity-seeking opportunist who has sought to profit from his involvement in the steroids scandal — to the point of wearing a company’s advertisement on his tie when he testified before the federal grand jury. McNamee donned the tie, Hardin said, because he knew he would be photographed leaving the courthouse.

McNamee even wrote an autobiography, which has not yet been published. Titled, “The Three Constants in Life: Death, Taxes and Mac,” the “x” in taxes was crafted using two syringes, according to a photograph of the book jacket that Hardin showed jurors.

“You cannot go anywhere with the government’s case unless you believe Brian McNamee,” Hardin added. “There is not going to be any real corroboration of Brian McNamee.”

On Monday, Assistant U.S. Attorney Steven Durham sought to pre-empt attacks on McNamee, telling jurors that they could trust the strength coach because authorities had taken steps to verify what he had told them.

Those efforts included using forensic tests to discover Clemens’s DNA on a syringe and cotton swabs that McNamee says he used to inject the pitcher with steroids. McNamee gave the syringe, which prosecutors say also contains steroid residue, to authorities through his lawyer.

But even that evidence came under assault by Hardin. He displayed a photograph of a crumpled Miller Lite can in which McNamee claims he stored the used syringe and cotton swabs. Hardin called the evidence “a mixed up hodgepodge of garbage” and said it would be “ludicrous to ever try to suggest that this is evidence of anything in a criminal case.” Hardin even suggested that McNamee manipulated the evidence before it made its way to authorities.

Hardin also showed jurors a map that illustrated the more than 75 locations federal officials visited or called to try to corroborate McNamee’s story. The display underscored the scope and cost of the government’s case, even though Hardin said he was not talking about “the government’s waste of resources.”

Whereas prosecutors have sought to establish a narrative about an aging superstar who turned to steroids to prolong his career, Hardin said baseball statistics indicate Clemens never took the drugs. The pitcher performed consistently during his 24 seasons in the big leagues — before he met McNamee in 1998, and after 2001, the last year in which McNamee has said he injected Clemens with steroids, Hardin said.

Clemens won his first Cy Young Award with the Boston Red Sox in 1986, and his seventh with the Houston Astros in 2004; he retired in 2007. Hardin credited Clemens’s success to hard work — not drugs.

“Using steroids and taking the cheap way out was a total anathema to everything he stood for,” the lawyer added.

Prosecutors called their first witness Tuesday as they sought to establish that the 2008 hearings were material to the work of lawmakers, a key element of Clemens’s charges of perjury, obstruction of Congress and making false statements. Clemens and McNamee both testified before the House Committee on Oversight and Government Reform.

Phil Barnett, the former Democratic staff director of the oversight committee, testified that the panel was looking into steroids in baseball because it was a significant public health concern, particularly if children emulated their athletic heroes and began taking the drugs. Barnett said the committee was interested in the issue because it was concerned that Major League Baseball was “lax” in policing steroid abuse.

During his questioning, prosecutors began playing excerpts of Clemens’s deposition by Barnett just before the trial broke for the day. It is in recess for the rest of the week and is scheduled to resume Monday.