The six-term senator retires in 2009, but not without leaving a historic legacy for 113 million Americans under employer-sponsored insurance plans who are expected to benefit from the legislation.
"This moves the ball tremendously," said Domenici. "I'm a pretty passionate guy, and there's a certain amount of pleasure in doing things that are hard to do."
Today, about 58 million Americans have a diagnosable mental disorder. Mental illness has also been associated with more than 30,000 suicides each year. An estimated 16 percent of all inmates suffer from mental illness.
The new law does not mandate group plans to provide mental health coverage, but requires insurance plans that offer mental health coverage to do so on a par with physical illnesses. Only companies with more than 50 employees must comply.
Before the 1980s, Domenici hadn't paid much attention to mental health care -- that is until his daughter, Clare, now 46, was diagnosed with atypical schizophrenia after her first year in college. She struggled with her focus, had bouts of anxiety and her personality began to change.
"For a long period of time, my parents were trying to determine what was the matter with her," younger sister Paula Domenici told ABCNews.com. "They were mystified."
Since then, her sister has gone on and off a cluster of medications – many with adverse side effects, such as weight gain.
"She had been a star athlete and a good student with lots of boyfriends," said Paula Domenici, 41, a psychologist who works with veterans in Bethesda, Md. "To see such a transformation was shocking. It's a slow evolution and sad for families to have to acknowledge."
"There is no simple solution," Paula said of mental illness. "It has a huge ripple effect and sadness that is tied to someone you love."
In the 1980s, when Domenici and his wife Nancy began attending a NAMI support group, they heard stories of families going broke, splitting up and mentally ill children ending up on the streets, in jail, or dead.
"I'd stop by the meetings on the way home from work and meet up with 10 or 12 parents," Domenici said. "They were so passionate and the government was not doing the right things."
After a few speeches here and there, he invited Wellstone – a Minnesota liberal – to help him push for health insurance parity.
They nurtured alliances, finding many others had their own touchstones. The niece of former Sen. Alan Simpson, R-Colo., and the father of Sen. Harry Reid, D-Nev., had committed suicide; former Vice-President Al Gore's wife Tipper had suffered from depression and Newt Gingrich, then the Republican speaker of the House, who didn't block passage of the bill, had a mother who suffered from bipolar disorder.
They passed a 1996 law that banned insurance companies from setting lifetime limits on mental health care treatment and teamed up again in 2001 on an earlier unsuccessful version of the 2008 law.
"They were good friends – my mom and Nancy [Domenici] and Pete and my dad," said Wellstone's son, David. "Even though they were politically worlds apart, they found some common ground on a personal level."
In October of 2002, Wellstone died at the age of 58, along with his wife Sheila and daughter Marcia, in a plane crash. His sons picked up their father's work with Kennedy and Sen. Mike Enzi, R-Wyo.
"It's been a roller coaster ride," Wellstone, now 43, told ABCNews.com. "I'm ecstatic."