Everyone knows that ideological polarization and rancorous partisanship have been on the rise in recent years. Who is to blame? Entranced by memories of the bipartisan compromises and chummy camaraderie that once prevailed in the nation's capital, prominent members of the Washington press corps like to blame both parties equally.

This, in turn, enrages liberals, who point out that it's Republicans who have been marching in lock-step opposition to the president's agenda from day one — and who turned the previously pro forma debt-ceiling vote into a game of high-stakes brinksmanship designed to extort policy concessions that are supported by few outside the Tea Party.

Fair enough.

But that doesn't mean liberals don't make their own unique contributions to polarization.

I'm not referring to liberal policy positions, which haven't shifted significantly leftward under Barack Obama.

I'm talking, instead, about Democrats placing ideological or partisan commitments ahead of principles they've upheld in the past and that they wouldn't hesitate to invoke against Republicans if the situation were reversed. Call it a proliferation of liberal double standards. Here are the three biggest:

1. Nullification For Me But Not For Thee. Going back to the early days of the republic, some have argued that states should have the power to nullify federal laws when the state considers those laws to be unconstitutional. John C. Calhoun was the greatest theorist and champion of this view, which played an important role in convincing slave states that they were justified in seceding from the Union. It was later used to legitimate segregation. Today it is most often invoked by conservatives who contend that states should be exempted from having to participate in implementing ObamaCare.

Others have taken an anti-nullification position, insisting that federal law trumps state law, and that the Supreme Court stands as the highest authority in judging the constitutionality of all laws, state and federal. This was the view espoused by Alexander Hamilton and Abraham Lincoln (the latter of whom was willing to wage the Civil War in order to settle the issue). Roughly half a century after opponents of the civil rights movement took a final, failed stand in favor of nullification, federal supremacy is finally the consensus view, accepted by many (though certainly not all) Republicans and virtually all Democrats.

Except, it seems, when it comes to state marijuana laws. While liberals normally treat arguments in favor of nullification as a cultural atavism, they appear to be perfectly content to permit Colorado and Washington to flout federal laws criminalizing pot.

Technically speaking, this might not be an example of nullification, since neither state claims to be legalizing pot as a protest against the unconstitutionality of federal anti-marijuana statutes. But the effect is the same. Why is it acceptable to permit states to countermand federal law when it comes to buying, distributing, selling, and smoking pot, but not okay for states to seek exemption from the raft of regulations imposed by ObamaCare? Just because liberals like the ACA and think smoking pot is no big deal?

2. Work Stoppage At The Justice Department. In states around the country, attorneys general are refusing to defend state-level laws and constitutional bans on gay marriage when they are challenged in court. And now Eric Holder, the nation's top law enforcement official, has come out in support of this selective strike by his underlings.

His reason? "If I were attorney general in Kansas in 1953, I would not have defended a Kansas statute that put in place separate-but-equal facilities." I wouldn't have either. But I would have understood that in refusing to do my job, I had committed a fireable offense.

Attorneys general are supposed to enforce democratically enacted laws, and they obviously have no independent authority either to pass or rescind laws themselves, so I have no idea how Holder can possibly justify his position. However noble the stand may seem to many gay-marriage supporters, it is a serious assault on the rule of law. In our system, the people make the laws through their elected representatives and judges decide whether or not the laws are constitutional. Government lawyers don't, and shouldn't, have the power to play either role.

This is something that I trust no liberal would deny at the level of principle. But if a cherished partisan cause can be furthered by selectively setting the principle aside? To judge by the silence with which liberals have greeted Holder's remarks, that's apparently no problem at all.

3. Liberal McCarthyism. Joseph McCarthy was a thug and a demagogue who abused his senatorial powers to persecute private citizens for their constitutionally protected political beliefs. Liberals are right to treat his crusade as a dark moment in the history of American civil liberties, and they have been fully justified in using his name as an epithet to keep public figures from deploying similar tactics against their political enemies.

But then what should we make of the absence of liberal outrage about Harry Reid's rant against right-wing industrialists Charles and David Koch on Feb. 26? Taking to the floor of the U.S. Senate, the majority leader denounced the Kochs for funding ads attacking ObamaCare and called on the American people to speak out against the "terrible dishonesty of these two brothers, who are about as un-American as anyone that I can imagine."

The Koch brothers spend many millions of dollars a year to make life difficult for Democrats, so liberals are entitled to their enmity. But the Kochs are exercising their constitutionally protected rights. As far as I know, there is no evidence that they have violated a single law in using their vast fortune to influence political debate. Until evidence of such illegality surfaces, Democrats should devote themselves to changing the laws that empower the Kochs and to fighting the political battle of ideas on its own terms. They should also demand that the party's most powerful officeholders not hurl slurs at private citizens.

In case readers suspect me of harboring concealed partisan preferences of my own, I'd like to close by making my positions completely clear: I have no objection to legalizing marijuana; I am in favor of gay marriage; and I have no sympathy at all for the anarcho-capitalism favored by the Koch brothers.

On these issues, at least, I am a liberal.

But my loyalty to the Constitution, the rule of law, and norms of democratic civility trumps my devotion to any partisan cause.

If only more liberals felt the same way.