Representative Stephen Lynch, the South Boston Democrat now running to succeed John Kerry in the Senate, voted against ObamaCare on the make-or-break House vote in March 2010. He was the sole member of the Massachusetts delegation to oppose the bill, and he did so in the face of personal entreaties by President Obama, by then-House Speaker Nancy Pelosi, and even by the widow of Senator Ted Kennedy, who had died just a few months earlier. He did so even though it angered many of his labor-union allies, and despite the president’s enormous popularity in Massachusetts. In the end Lynch was one of just 34 Democrats in Congress – and the only one in New England – to vote no.

Before long he was facing a serious re-election challenge within his own party, the first since he was elected to the House a decade earlier. (The Boston Globe, which had warned Lynch against making “a grievous error” by voting no on ObamaCare, endorsed his opponent in the Democratic primary that fall.) Now, as Lynch and fellow Representative Ed Markey – who says passing the health law was “one of the most important votes of my career” – compete for their party’s nomination in the Senate race, that 2010 vote is back in the spotlight.

What do you call it when a congressman opposes a bill it would be far easier to support, infuriating much of his political base and putting his electoral prospects at risk? Richard Kirsch, a key strategist for the progressive coalition that spent $47 million to get ObamaCare passed, has been calling it “cowardice.” I do not think that word means what he thinks it means.

Whether or not you find Lynch’s arguments against the Affordable Care Act persuasive, it took a certain amount of backbone to buck his party and vote no. “All of us in the Congress,” an earlier Massachusetts lawmaker once wrote, “are made fully aware of the importance of party unity … and the adverse effect upon our party’s chances in the next election which any rebellious conduct might bring.”

That lawmaker was John F. Kennedy, and those words are from the first chapter of Profiles in Courage, which describes the “terrible pressures” that discourage most elected officials from acts of political courage. Like all Americans, said JFK, politicians “prefer praise to abuse, popularity to contempt.” They also face the pressure of getting re-elected, and the pressure from interest groups and organized constituents.

So it’s understandable that many of them “tend to take the easier, less troublesome path” and find a way to “rationalize what first appears to be a conflict between their conscience … and the majority opinion of their constituents,” Kennedy wrote. Most politicians have “developed the habit of sincerely reaching conclusions inevitably in accordance with popular opinion.”

It’s a wonderfully convenient habit, as political figures right and left have recently been demonstrating.

Hillary Clinton last week became the latest national politician to “evolve” into a supporter of same-sex marriage, something she had always publicly opposed. Her new position she ascribed not to polls showing majority support for gay marriage, or to the prospect of another presidential campaign, but to her “devotion to law and human rights and the guiding principles of my faith.”

Clinton’s switch came several days after Ohio Senator Rob Portman reversed his longtime opposition to same-sex marriage. What prompted his change of mind, he wrote in a column, was learning two years ago that his son is gay. Now he favors same-sex marriage, Portman explained, because he is a conservative, not in spite of it.

Other politicians, meanwhile, are “evolving” on immigration. The latest is Senator Rand Paul, the Kentucky Republican who has been such an immigration hardliner that in 2011 he sponsored a constitutional amendment to deny American citizenship to the US-born children of illegal immigrants. But in a speech last week to the Hispanic Chamber of Commerce, he was singing from a different hymnal, reassuring millions of unlawful immigrants that if they want to work in America, “then we will find a place for you.”

Maybe all the marriage and immigration evolvers – and all the ones still to come – are perfectly sincere. Or maybe they’re seeing the light only because they’re feeling the political heat. “Those are my principles,” Groucho Marx is quoted as saying. “And if you don’t like them, I have others.” Politicians have a remarkable capacity for changing their deeply-held views when it’s in their political interest to do so. Those who do it often and clumsily, like the last Republican and Democratic presidential candidates from Massachusetts, earn a reputation as flip-floppers. But virtually all of them do it.

Rare is the politician who takes a stand he knows may doom him politically, defying party or public opinion as a matter of principle. JFK’s celebrated book about eight of them won the Pulitzer Prize in 1957. Lynch’s ‘no’ vote on ObamaCare wasn’t of that lofty caliber. But it showed a measure of grit that more politicians ought to cultivate.

(Jeff Jacoby is a columnist for The Boston Globe. His website is www.JeffJacoby.com).