McCain aside, the most common last-ditch argument for preserving “don’t ask” heard last week, largely from Southern senators, is to protect “troop morale and cohesion.” Every known study says this argument is a canard, as do the real-life examples of the many armies with openly gay troops, including those of Canada, Britain and Israel. But the argument does carry a telling historical pedigree. When Harry Truman ordered the racial integration of the American military in 1948, Congressional opponents (then mainly Southern Democrats) embraced an antediluvian Army prediction from 1940 stating that such a change would threaten national defense by producing “situations destructive to morale.” History will sweep this bogus argument away now as it did then.

Image Frank Rich Credit... Fred R. Conrad/The New York Times

Those opposing same-sex marriage are just as eager to mask their bigotry. The big arena on that issue is now in California, where the legal showdown over Proposition 8 is becoming a Scopes trial of sorts, with the unlikely bipartisan legal team of David Boies and Ted Olson in the Clarence Darrow role. The opposing lawyer, Charles Cooper, insisted to the court that he bore neither “ill will nor animosity for gays and lesbians.” Given the history of the anti-same-sex marriage camp, it’s hard to make that case with a straight face (so to speak). In trying to do so, Cooper moved that graphic evidence of his side’s ill will and animosity be disallowed  including that notorious, fear-mongering television ad, “The Gathering Storm.”

The judge admitted such exhibits anyway. Boies also triumphed in dismantling an expert witness called to provide the supposedly empirical, non-homophobic evidence of how same-sex marriage threatens “procreative marriage.” In cross-examination, Boies forced the witness, David Blankenhorn of the so-called Institute for American Values, to concede he had no academic expertise in any field related to marriage or family. The only peer-reviewed paper he’s written, for a degree in Comparative Labor History, was “a study of two cabinetmakers’ unions in 19th-century Britain.”

In another, milder cross-examination  on “Meet the Press” last weekend  John Boehner, the House G.O.P. leader, fended off a question about “don’t ask” with a rhetorical question of his own: “In the middle of two wars and in the middle of this giant security threat, why would we want to get into this debate?” Besides Mullen’s answer  that it is the right thing to do  there’s another, less idealistic reason why President Obama might want to get into it. The debate could blow up in the Republicans’ faces. A protracted battle or filibuster in which they oppose civil rights will end up exposing the deep prejudice at the root of their arguments. That’s not where a party trying to expand beyond its white Dixie base and woo independents wants to be in 2010.

Polls consistently show that independents, however fiscally conservative, are closer to Democrats than Republicans on social issues. (In May’s Gallup survey, 67 percent of independents favored repealing “don’t ask.”) This is why Scott Brown, enjoying what may be a short-lived honeymoon in his own party, calls himself a “Scott Brown Republican.” A Scott Brown Republican isn’t a Boehner or Hatch Republican. In his interview with Barbara Walters last weekend, he distanced himself from Sarah Palin, said he was undecided on “don’t ask” and declared same-sex marriage a “settled” issue in his state, Massachusetts, where it is legal.

It’s in this political context that we can see that there may have been some method to Obama’s troublesome tardiness on gay issues after all. But as we learned about this White House and the Democratic Congress in the health care debacle, they are perfectly capable of dropping the ball at any moment. Let’s hope they don’t this time. Should they actually press forward on “don’t ask” in an election year with Mullen and Gates on board  and with even McCain’s buddy, Joe Lieberman, calling for action “as soon as possible”  they could further the goal and raise the political price for those who stand in the way. Recalcitrant Congressional Republicans will have to explain why their perennial knee-jerk deference to “whatever the commanders want” extends to Gen. David Petraeus and Gen. Stanley McChrystal on troop surges but not to Mullen, who outranks them, on civil rights.

The more bigotry pushed out of the closet for all voters to see, the more likely it is that Americans will be moved to grant overdue full citizenship to gay Americans. It won’t happen overnight, any more than full civil rights for African-Americans immediately followed Truman’s desegregation of the armed forces. But there can be no doubt that Mike Mullen’s powerful act of conscience last week, just as we marked the 50th anniversary of the Greensboro, N.C., lunch counter sit-in, pushed history forward. The revealing silence that followed from so many of the usual suspects was pretty golden too.