For the elder Wellstone, passion arose from contact with constituents who'd lost children to anorexia and suicide – and a college-age brother who had a mental breakdown when the senator was 10.
"My father visited his brother [in a mental institution] and saw the deplorable conditions and the toll it took on the family emotionally and financially," said Wellstone's son.
He saw it as a "civil rights issue," said his son of his father. "He was tireless. He was a heck of a role model. He walked the talk."
Domenici says that even with this new legislation, the health-care system is broken in many respects, but he is wary of reforms that would put government in charge of Americans' health care. Still he favors federal support of more facilities to house the growing number of mentally ill.
There's also the legions of uninsured who won't be helped by the Wellstone-Domenici bill.
"This doesn't cover everything," Domenici said, referring to the 47 million American uninsured for whom the law will not apply. "But that will come."
Overall, the parity law is expected to cost the federal government $3.4 billion over the next 10 years, because employers will have more health expenses that they can deduct from their income taxes. It will also increase the costs to private insurers and possibly lead to higher premiums, though the total bill is unclear.
Domenici will be "irreplaceable" in the Senate according to NAMI director Fitzpatrick, who hailed the law for de-stigmatizing the plight of the mentally ill.
The senator's daughter Paula admires the focus and passion that has marked her father's 20-year devotion to an issue that started at home.
"When my dad sees a true injustice and knows the facts, he's 150 to 200 percent dedicated to persevere to correct it," she said. "I don't know if it's from the tradition of his Italian family, or his Catholic faith or being a lawyer, but it makes him tenacious and persistent until he rights the wrong."
Suzanne Bernard of ABC News Research contributed to this story.