Mitt Romney and John Kerry have long sat on opposite ends of a political seesaw.

If Kerry had become president, his victory would have hurt Romney’s presidential prospects. If Romney wins this year, Kerry’s hopes of becoming secretary of state will be dashed.

Their teeter-totter relationship dates from Romney’s early months as governor. Although Romney developed a solid working relationship with Ted Kennedy, nothing similar emerged with Kerry, who was planning a presidential campaign even as Romney took office.

Had Kerry won the White House, his victory would have made it more difficult for Romney to find traction for the 2008 campaign that put him in the primary pole position this time around. Although Romney could easily have supported George W. Bush in 2004 without lacing into Kerry, he instead emerged as a particularly acidic critic. In an appearance with President George W. Bush, Romney maintained that Kerry “has been on both sides of most issues” and offered a litany of would-be examples.

In another speech, Romney painted Kerry as an inveterate flip-flopper, portraying him as suffering from something close to a psychological flaw. “For those who don’t understand how he can be so vacillating, it stems from the fact that he’s very conflicted,” Romney said. “If he’s with an audience, he wants to identify with and satisfy that audience, and will say what he thinks they want to hear. And if that audience, for instance, is on one side of an issue he’ll follow that, on another, he’ll follow another.” (Those comments are amusing now, given the reputation for ideological acrobatics that Romney subsequently acquired as he flip-flopped his way into conservative conformity.) Over the years, Kerry’s two marriages to rich women, capacious home, and wealthy lifestyle have also been targets of Romney’s mockery.

Kerry’s 2004 defeat didn’t end his presidential ambitions; the senator hoped to run again in 2008. Then a joke in a 2006 speech to California college students misfired. Kerry meant to tweak President George W. Bush by warning students that if they didn’t study hard, “You end up getting us stuck in a war in Iraq.” Instead, the punch line came out this way: “You get stuck in Iraq.” Now, one would have to be a dupe or a dope to think that a politician harboring national aspirations would intentionally insult US troops. Yet conservative outrage-mongers pretended to think just that — and Romney, now actively planning his 2008 presidential run, quickly joined in, demanding that Kerry apologize.

“When I saw his comments, I was just astounded,” said Romney. “What he said was offensive.”

That controversy killed Kerry’s already iffy (second) chances. These days, the long-time Democratic senator is hoping to cap his career with a stint as secretary of state in a second Obama term. That obviously won’t happen if Romney becomes president — something Kerry seems determined to prevent.

The Democratic senator has delighted in scolding Romney on foreign policy. In March, he took Romney to task for his “blisteringly inaccurate” assertions about the New Strategic Arms Reduction Treaty, which Kerry said were designed to sharpen “his newly minted conservative credentials.” He then castigated Romney for a politically motivated stand on Iran “that is as inaccurate as it is aggressive.” In May, he called Romney’s characterization of Russia as our number one geopolitical foe “breathtakingly off target and naive,” criticized his gubernatorial record, and slammed his economic ideas.

Kerry has also signed on to play Romney in practice debates with Obama. This week, after Romney and a spokesman cited Kerry and wife Teresa Heinz as a rationale for only releasing two years of tax returns, Kerry chief of staff David Wade rebuked them in an online piece, noting that Kerry, who files separately from his wife, had released 20 years of returns over his public career.

The Kerry camp downplays Kerry’s anti-Romney role, saying the senator is simply offering necessary perspective on Romney’s statements and correcting the record.

Ah, but of course. And yet, here’s one safe bet: Kerry will feel the need to do more and more of that as the campaign continues.