NEW YORK (FORTUNE) - How much do closing arguments really matter?

It's hard to escape the feeling that the Enron jury has already made up its mind, after fifteen weeks of trial and 54 witnesses, plus 14 days of testimony from the two defendants, former Enron CEOs Ken Lay and Jeff Skilling.

Special Reportfull coverage Skilling gets 24 years Lessons from Enron: Just say 'sorry' Skilling's possible new home Skilling sentence likely to be a long one Meet the players Find out who you might have seen at the Enron trial, how they got involved, and what they're doing now. Launch gallery

More from Fortune Will Mmmhops be a hit? NBA confirms L.A. Clippers sale to ex-Microsoft CEO Steve Ballmer FBI and SEC probe into Carl Icahn and golfer Phil Mickelson FORTUNE 500 Current Issue Subscribe to Fortune

Having said that, conventional legal wisdom holds that closing arguments can crystallize the evidence and give the jurors something to grab onto during their deliberations - talking points during their private debate.

Under the rules of engagement for this trial, prosecution and defense are each getting six hours to make their final case. The government went first Monday; lawyers for Lay and Skilling will divide their time on Tuesday; and the government will get final word, with a short rebuttal on Wednesday that should send the case to the jury for final deliberations by lunchtime.

On Monday, it was prosecutor Kathy Ruemmler's job to sum up the government's charges and evidence. This was not a small task, and Ruemmler consumed precisely three hours and 51 minutes (leaving two hours and nine minutes for Enron Task Force Director Sean Berkowitz on Wednesday), in a presentation that sounded the government's theme from the start: This is not a case about accounting, but about lies and choices.

"Mr. Skilling and Mr. Lay lied over and over and over again," Ruemmler declared. "....Those lies were criminal."

Facing the closing-argument challenge of a lifetime, Ruemmler, 35, had started off a little nervous, seeming unsure where or how to stand - fold your arms or keep them at your side? pace before the jury or stand still at the lectern? - before settling into a deliberate rhythm. In fact, it was a little too deliberate.

By the lunch break, she was running behind schedule, having bogged down in details about Skilling's alleged crimes surrounding Enron Energy Services, the company's troubled retail business. After lunch, she tried to make up for the wide swath of territory that remained to traverse, rushing through details, before again settling down.

Ruemmler threw the defendants' own words back at them, putting up a giant poster containing Lay's earlier testimony that while rules are important, "you should not be a slave to the rules either."

She scoffed at defense claims that Enron was a healthy company sent into a tailspin by a conspiracy between short-sellers and the media ("it's absurd; it's ridiculous; don't buy it!"); blasted Skilling for convenient memory lapses; and accused both men of continuing to spin an elaborate cover story ("losses aren't really losses; stock sales aren't really stock sales; and incriminating conversations are really just innocent misunderstandings.")

"You lie; you ridicule; you call someone a profane name," Ruemmler summed up. Lay and Skilling, she declared, were engaging in "the exact same tactics" that they employed while running Enron. But "in this courtroom, ladies and gentlemen, Ruemmler added, "the cover stories have been blown. You're heard the truth."

Ruemmler's best moments came when she stepped back and urged the jury to weigh the evidence with common sense. Why had so many government witnesses told the story so differently from Lay and Skilling? They weren't exactly career criminals - was it really plausible to believe that all the government "cooperators" had succumbed to pressure and made their stories up? Why did so many e-mails and memos support the tale they had told?

Through it all, the defendants mostly looked on solemnly, though Skilling on occasion smiled or shook his head in seeming contempt at Ruemmler's statements.

Ruemmler predicted that the defense lawyers would continue to blame Fastow for Enron's woes, but noted that while they readily cited his (admitted) schemes to line his own pockets, they simultaneously denied his (also admitted) schemes to commit fraud to help cook Enron's books.

"What they would have you believe," Ruemmler noted, "is when he was CFO of Enron, he was pure as the driven snow. Fraud on the side - no fraud at Enron?"

She raced through a summary of deceptive statements about Enron's broadband business, then its use of reserves to add a penny or two of reported earnings to meet analysts estimates ("abracadabra!....that's fraud! That's wrong!"). Then she turned to Lay's rosy assurances to investors and employees after returning as CEO following Skilling August 2001 resignation.

Ruemmler zeroed in on Lay's public insistence that there was "no other shoe to fall" at Enron - even after he had privately received warnings about an array of growing problems. She noted that Lay had buttressed his reassurances by giving the public the impression that he was personally a "net buyer" of Enron shares.

Of course, she then noted that Lay was actually secretly selling $70 million in Enron shares - almost two-thirds of his shares - "selling stock back to the company in a way that no one can detect." Noted Ruemmler: "What he did was he took care of himself."

On the witness stand, Lay had bristled that his personal stock sales weren't part of the criminal indictment. Ruemmler addressed this too, noting that Lay had misled the public by publicly declaring he was buying Enron shares - and encouraging them to do the same.

"He didn't have to say that," she noted. "That's why we're talking about his stock sales." Added Ruemmler: "People had the impression that when Mr. Lay said he was buying stock - I don't know, call us all crazy - that he was buying stock!"

Rushing to finish, Ruemmler ended her argument in classic fashion: by urging the jury to hold Lay and Skilling "accountable for the choices that they made and the lies that they told. Find them guilty on all counts."

Tuesday's closing arguments, on behalf of the two defendants, should be longer on flash and bombast.

It starts with Skilling's silver-tongued lead attorney Dan Petrocelli, who has 3 hours and 20 minutes (Skilling faces far more charges than Lay). In an extremely unusual move, four Lay defense lawyers will tag-team the remaining two hours and forty minutes, with the headliner being lead attorney Mike Ramsey, back in the courtroom after missing most of the trial with heart problems.