And after a 4½-hour hearing, Congress settled for a draw in the he-said, he-said between the two men over whether the seven-time Cy Young Award winner used performance-enhancing drugs. Ultimately, the matter may be referred to the Justice Department for a resolution - and, possibly, criminal charges.
"I haven't reached any conclusions at this point," said California Democrat Henry Waxman, chairman of the House Oversight and Government Reform Committee.
But, as ranking Republican Tom Davis of Virginia, put it: "Both can't be telling the truth."
Clemens and McNamee, once employer and employee, and by all accounts once friends, sat at the same witness table and rarely, if ever, looked at the other.
His reputation and legacy on the line, Clemens gestured toward McNamee with his right arm and said, "I have strong disagreements with what this man says about me."
At times, Clemens struggled to find the right words as he was pressed by lawmakers about McNamee's allegations - told to federal prosecutors and then baseball investigator George Mitchell - that he injected the pitcher with steroids and human growth hormone from 1998 to 2001. Clemens also was asked about new accounts of drug use made against him by former teammate and pal Andy Pettitte.
Using words like "misremembered" and even mispronouncing McNamee's name at one point, Clemens raised his voice toward the end to interrupt Waxman's closing remarks. Waxman pounded his gavel and said, "Excuse me, but this is not your time to argue with me."
It was extraordinary theater, writes CBSNews.com legal analyst Andrew Cohen in the Couric & Co blog.
"The Zen moment of the day came early when Clemens told the Committee that Petitte had 'misheard' and 'misremembered' Clemens talking about his own steroid use. It is Petitte's new testimony - directly linking Clemens to an acknowledgement of the use of performance-enhancing drugs - that is probably the most likely to generate a perjury charge against Clemens. And his defense - that the government's star witness 'mis-remembers' - is not likely to gain a lot of traction before a judge or jury should we ever get to the unhappy prospect of a Clemens' perjury trial," writes Cohen.
It seemed clear nearly from the start that the committee would not treat Clemens with kid gloves, despite all the face-to-face sit-downs he did with representatives in recent days - sometimes posing for photos or signing autographs for staff members.
Later, the committee appeared split along party lines, with the Democrats reserving their most pointed queries for Clemens, and the Republicans giving McNamee a rougher time. Chris Shays, a Connecticut Republican, likened the hearing to a "Roman Circus" featuring gladiators.
"I have never taken steroids or HGH," Clemens said after rising with McNamee to swear to tell the truth. "No matter what we discuss here today, I am never going to have my name restored."
For many, his denials rang hollow.
"It's hard to believe you, sir," said Rep. Elijah Cummings, a Maryland Democrat. "I hate to say that. You're one of my heroes. But it's hard to believe."
McNamee's answers were generally quick and concise. His credibility also came under scrutiny.
"You're here under oath, and yet we have lie after lie after lie after lie," said Rep. Dan Burton, an Indiana Republican.
When it was over, Clemens left the hearing room through a back door. Just before exiting, he paused to shake hands with Davis. Clemens later spoke briefly to reporters, saying: "I'm very thankful and very grateful for this day to come. I'm glad for the opportunity finally. And, you know, I hope I get - and I know I will have - the opportunity to come here to Washington again under different terms."
Since both men can't be telling the truth, it may come down to physical evidence to see who committed perjury before Congress, reports CBS News investigative correspondent Armen Keteyian.
The best physical evidence against Roger Clemens, the "smoking guns", may be the bloody gauze pads, vials and syringes McNamee said he used to inject Clemens in 2000 and 2001.
A former police officer, McNamee told the committee he kept the material in waste-disposal box at home for years - for one reason: "While I liked and admired Roger Clemens, I don't think I ever really trusted him."
Federal authorities have reportedly sent the evidence to a lab for testing, setting up a potential forensic duel, reports Keteyian.
Exactly two months after the Mitchell Report was released, Clemens and McNamee were separated by one seat in the same wood-paneled room where Mark McGwire and Rafael Palmeiro saw their careers tarnished during a hearing in March 2005. In a reference to McGwire's evasions that day, Rep. Mark Souder, R-Ind., admonished Wednesday's witnesses by saying: "It's better not to talk about the past than to lie about the past."
Clemens briefly stared at McNamee, his former employee, during his accuser's opening statement, in which the trainer said he injected Clemens more times than he previously had said.
Members of Congress questioned the credibility of both.
Waxman - who opened the proceedings by saying he thought this would be the last hearing his committee holds on baseball - pointed out inconsistencies in Clemens' comments. Waxman also accused Clemens of possibly attempting to influence statements to the committee by the pitcher's former nanny.
Burton repeatedly read remarks McNamee had made, and each time the former trainer was forced to admit they were untrue.
"This is really disgusting. You're here as a sworn witness. You're here to tell the truth," he said. "I don't know what to believe. I know one thing I don't believe and that's you."
Cummings set the tone within minutes of the start, repeatedly reminding Clemens he was under oath and admonishing the pitcher to "keep your voice up." McNamee was asked to pull his microphone closer.
Debbie Clemens, the pitcher's wife, sat behind her husband and listened as Waxman implicated her in HGH use, citing statements by Pettitte. Later, Clemens read a statement from his wife and said she "has been broken up over this."
IRS Special Agent Jeff Novitzky, a key member of the federal prosecution team against Barry Bonds, watched from a second-row seat. Asked why he was there, Novitzky declined comment.
Bonds, baseball's home run king, was indicted in November on charges of perjury and obstruction of justice stemming from his 2003 testimony to a grand jury in which he denied knowingly using performance-enhancing drugs. Because of his denials under oath, Clemens could be subjected to a similar criminal probe. McNamee also could be referred to the Justice Department.
Pettitte, who was excused from testifying, said in a statement to the committee that Clemens admitted to him as long as 10 years ago that he used HGH. Waxman read from affidavits by Pettitte and Pettitte's wife, Laura, supporting the accusations.
"Andy Pettitte is my friend. He was my friend before this. He will be my friend after this and again. I think Andy has misheard," Clemens said. "I think he misremembers."
McNamee told Mitchell that he injected Clemens 16 to 21 times with steroids and human growth hormone from 1998-01, and that Pettitte and Chuck Knoblauch used HGH. In his opening statement, McNamee said he might have injected Clemens and Knoblauch more than that.
"I have helped taint our national pastime," McNamee said. "Make no mistake: When I told Sen. Mitchell that I injected Roger Clemens with performance-enhancing drugs, I told the truth."
Waxman said McNamee, a former New York City police officer, lied to police seven years ago during an investigation of a possible rape. He also was tough on Clemens.
"We have found conflicts and inconsistencies in Mr. Clemens' account. During his deposition, he made statements that we know are untrue," Waxman said.
In the affidavit, Pettitte said Clemens backtracked when the subject of HGH came up again in conversation in 2005, before the same House committee held the first hearing on steroids in baseball.
Pettitte said in the affidavit that he asked Clemens in 2005 what he would do if asked about performance-enhancing substances. Pettitte said Clemens responded by saying Pettitte misunderstood the previous exchange in 1999 or 2000 and that, in fact, Clemens had been talking about HGH use by his wife in the original conversation.