(FORTUNE Magazine) – IT'S HURRICANE SEASON ON THE GULF Coast, and two weeks into the first Vioxx personal-injury trial in Angleton, Texas, drugmaker Merck is facing a category 5 force of nature: W. Mark Lanier, a bigtime plaintiffs lawyer who also happens to be an evangelical preacher. In his opening statement, Lanier, 44, spoke for 2½ hours without notes, unleashing a frighteningly powerful, spellbinding attack upon the company. He spoke in gloriously plain English--gastrointestinal ulcers are "tummy bleeds" in his lexicon--and he accompanied nearly every sentence with imaginative if hokey PowerPoint slides and overhead projections. Lanier represents Carol Ernst, whose marathoner husband, Bob, died of a heart arrhythmia at 59 in May 2001. Lanier says the death was caused by Vioxx. He showed the jury photos of Bob and Carol at a kite festival, Bob and Carol at a tandem-bike race, and finally Bob hugging Carol by a lake. "Then things changed," Lanier said, and Bob's image was suddenly whited out of the photo--replaced with an empty outline of his silhouette. "Bob died."

Lanier spun out a storybook tale of a drug company hijacked in 1994 by a "businessman CEO" named Ray Gilmartin (a slide depicts Harvard Business School on the Charles), who turned the formerly good company into an ATM for "spitting out money." Lanier appealed to the jurors to join him on a bracing mission: "You've got to get us to justice," he said (slide shows a gavel lying across an American flag).

For his first witness, he called Merck epidemiologist Nancy Santanello. What, he asked, would have been the "downside" to warning the public about an early study that showed that patients taking Vioxx had five times as many heart attacks as a group taking naproxen (Advil). "There's none," she responded, "and I'm sure we did." But then Lanier played for her one of those famous Dorothy Hamill TV commercials for Vioxx. When the voiceover listed the warnings, Lanier scratched them down on a big tablet of chart paper. "Y'all warned about allergic reactions, asthma, stomach bleeds, liver, and kidney," said Lanier. "Did you hear anything about cardiovascular events?" There had been no such warning. Nor was there any in the next commercial he played. Or the next. Or the next.

What makes the Ernst case so important is that it may help determine whether the Vioxx debacle turns into the biggest pharmaceutical mass tort in history. At last count Merck faced 4,275 Vioxx cases in state and federal courts around the country, and a New Jersey state judge has predicted that the tally may eventually top 100,000. Analysts' predictions of Merck's liability range from $2 billion to $20 billion, but no one really has a clue until verdicts in the early barometer trials, like Ernst's, start attaching pricetags to the cases. That's what drew some 200 lawyers, Wall Street analysts, and journalists to the gray, stone-block Brazoria County courthouse, braving the heat and humidity that fogged their glasses whenever they stepped out of their air-conditioned rental cars.

Born to a religious family of humble means in Lubbock, Lanier began preaching in his teens. He also became a champion debater, making it to the national finals, where he argued before Ken Starr, he recalls during an interview with FORTUNE at a well-chosen Italian restaurant in Houston, not far from his war-room at the Four Seasons hotel. (Lanier is a teetotaler, but his denomination is not otherwise ascetic. Invitations to his opulent Christmas barbecues--where stars like Diana Ross have performed--are coveted among local lawyers and judges.) Lanier estimates that he had given some 2,000 speeches by the time he finished high school. He got a seminary degree from a religious college in Tennessee, then graduated from Texas Tech Law School (tuition: $4 per course-hour) at age 23.

During jury selection Lanier mentioned his preaching sideline three times. When a Merck lawyer asked the prospective jurors whether Lanier's background would give him any advantage in their eyes, one now-impaneled woman admitted it might, explaining, "He's a preacher--'nuff said!"

Even if Merck's conduct is hard to defend, Lanier must still prove "causation"--i.e., that Vioxx caused the plaintiff's particular heart attack. That's difficult, since coronaries are so common. Merck will be in great shape, then, if they can get jurors to compartmentalize their thinking the way, say, constitutional-law professor Laurence Tribe does. Trouble is, such jurors don't exist. -- Roger Parloff