And he won Mitt Romney's vote in the 1992 Massachusetts presidential primary.
"This land, this water, this air, this planet, this rain, this is our legacy to our young, yet the Reagan-Bush years have been a time of cynical avoidance of one environmental issue after another - acid rain, energy conservation, depletion of the ozone layer, global warming and uncontrolled world population," Tsongas, a former U.S. senator from Massachusetts, said when he announced his presidential campaign in April 1991.
"Journey with me to a true commitment to our environment," he asked would-be supporters.
That October at the National Press Club, Tsongas argued that Bush's pandering to the "right wing" risked environmental Armageddon.
"You just cannot keep on growing the population in this world and not expect Armageddon to take place," Tsongas said.
"And this is where a George Bush, I think, is cynical to a degree that is not appreciated," said Tsongas. "George Bush goes from Planned Parenthood to anti-choice to placate the Republican right wing. Not only does that result in millions of American women having their rights taken away from them, potentially when Roe v. Wade is struck down, but it also means that under Reagan-Bush that our mandate to the international agencies is to pull back on population. So it means that you have this ever-expanding world population with a finite resource base in the world."
"I think that is morally reprehensible," said Tsongas. "So I believe that controlling world population is the No. 1 environmental issue."
In late January 1992, at a forum sponsored by Jesse Jackson's Rainbow Coalition, Tsongas accused Bush of having no "core" - as evidence by his reversal on abortion.
"And for George Bush to go from Planned Parenthood to anti-choice, that's quite a journey, folks. People who have cores can't walk that," said Tsongas. "And it's not only that it affects millions of women in this country, but in the world. The lack of population control means that there's no environment that is sustainable in this earth."
"I will tell you very strongly the No. 1 environmental issue I'm going to push for when I'm president is population control around this world so we can turn to later generations and say something except, 'Sorry, folks,'" Tsongas vowed.
Two months later, Romney cast his vote for Tsongas.
That Massachusetts primary was a landslide in both parties. Bush beat Pat Buchanan (who I served as research and issues director that year) 66 percent to 28 percent. Among Democrats, native son Tsongas took 66 percent to then-former California Gov. Jerry Brown's 15 percent and Arkansas Gov. Bill Clinton's 11 percent.
Two years later, when he announced he would seek the Republican Senate nomination to challenge Ted Kennedy, Romney told the Boston Globe about his vote for Tsongas.
"Romney confirmed he voted for former U.S. Sen. Paul Tsongas in the state's 1992 Democratic presidential primary, saying he did so both because Tsongas was from Massachusetts and because he favored his ideas over those of Bill Clinton," the Globe reported on Feb. 4, 1994. "He added he had been sure the GOP would renominate George Bush, for whom he voted in the fall election."
Romney's vote for Tsongas came up again in a profile the Globe published Aug. 7, 1994.
"Like his father, he wasn't a strong party man," the Globe said. "He had been a registered independent all his life. He still was, as he pondered the Kennedy challenge. He had even voted for Paul Tsongas in the 1992 Massachusetts Democratic presidential primary."
When the Los Angeles Times mentioned the Tsongas vote in a profile published Oct. 7, 1994, it did so in the context of Romney's wife pointing out that Romney had considered running for the Senate as an independent.
"When Romney decided to run, Republicans exchanged quizzical looks: 'We didn't know a single Republican when we jumped in in December,' his wife, Ann, says," the Times reported.
"As a registered independent, Romney had voted in the Democratic presidential primary in 1992 to support Paul E. Tsongas (though he backed George Bush in the general election, he says)," the Times reported. "He briefly considered running for the Senate seat as an independent, as well, his wife says, before rejecting the idea as impractical."
Thirteen years later, when Romney was seeking the 2008 Republican presidential nomination, he appeared on ABC's "This Week With George Stephanopoulos." Stephanopoulos asked him about the Tsongas vote.
Now, Romney said he did it because he wanted the Democrats to nominate the weakest candidate.
"When there was no real contest in the Republican primary, I'd vote in the Democrat primary, vote for the person who I thought would be the weakest opponent for Republican," Romney said.
"I'm a Republican and have been through my life," Romney said. "I was with Young Republicans when I was in college back at Stanford. But a registered independent, so I could vote in either primary."
And that is the core of his explanation.