BOSTON — As if the Republicans weren’t having enough trouble with defectors, they’ve gone on a purge. There was Dick Cheney on “Face the Nation.” Asked to pick between a GOP like Rush Limbaugh or Colin Powell, the former Veep not only chose Rush but snarkily crossed the general off the party list, saying, “I didn’t know he was still a Republican.”

This was less than two weeks after Arlen Specter assessed the odds of winning a Republican primary in Pennsylvania at exactly zip. Even the man who bullied Anita Hill and still favors Norm Coleman over Al Franken was pushed to the other side of the aisle.

The not-so-fond farewells that pursued Specter were nothing compared to the GOP un-eulogies for David Souter. The justice who had been anointed by Republican Sen. Warren Rudman, nominated by Republican George H.W. Bush, and confirmed with overwhelming Republican Senate support was excommunicated as a faux member of the party.

You will not be surprised at what these three purgees have in common: They are all supporters of a woman’s right to choose. Nor will you be surprised that abortion is the purity test for remaining in the GOP inner circle. Indeed, this test will be given again in the wrangling over Souter’s replacement. Unless President Obama nominates Robert Bork to the Supreme Court, we are in for another battle centered, again, on Roe v. Wade.

We forget sometimes that the GOP was not always like this. Not that long ago, we had pro-choicers like Barry Goldwater. Or Ronald Reagan, Act One. Or George H.W. Bush, who was pro-choice before he was pro-victory. The northeast quadrant of the country was once home turf to pro-choice Republican women from Planned Parenthood founder Margaret Sanger to Barbara Bush. Now we are down to two female Republican senators from Maine who may yet be banned from taking communion with their fellow congressional parishioners.

This purge has led me to wonder what would have happened if the first abortion case to arrive at the Supreme Court were not Roe v. Wade. What if it had been Susan Struck v. Secretary of Defense? What if it had been brought by the woman who did not want an abortion?

Lately, Justice Ruth Bader Ginsburg has been traveling, showing her mettle and health, talking about the loneliness of the sole female justice on a bench with eight brothers. On several occasions she mused out loud about the case that got away — the one she would have liked to argue before the court back when she was a women’s rights litigator.

Susan Struck was an Air Force captain, a nurse, when she got pregnant in 1970. Her commanding officer said she had two choices: abortion or resignation. Struck picked a third choice: a lawsuit. Ginsburg, then an ACLU lawyer, argued that the regulation banning pregnant women from service was sex discrimination. She also argued that Struck’s right to bear or not bear a child was her personal choice. Government intervention was a violation of her liberty.

The case was heading to the Supreme Court when Solicitor General Erwin Griswold — former head of Harvard Law School, where he treated female students with disdain — figured that he was going to lose. So the savvy solicitor advised the armed services to change the rules and the case became moot.

Today, it is mind-bending to think about how different the whole debate might have been if the first Supreme Court case arguing for the right to decide had been brought by a woman wanting to have a baby. Would we have better understood this reality: a government that can force a woman to have an abortion is the same government that can force a woman to continue the pregnancy? Would it have changed a Republican Party that was traditionally so wary of government power-grabs?

It’s all history now. But when the argument over the next justice comes, when you hear about Jane Roe, think of Susan Struck. When “pro-choice conservative” sounds like an oxymoron, remember the words of “Mr. Conservative,” Barry Goldwater:

“A lot of so-called conservatives today don’t know what the word means. They think I’ve turned liberal because I believe a woman has a right to an abortion. That’s a decision that’s up to the pregnant woman, not up to the pope or some do-gooders on the religious right. It’s not a conservative issue at all.”

Where would he be today? What would Dick Cheney say? Barry, get thee to a gulag.