The banking lobby plans to object to the Obama administration's bank tax proposal on the grounds that it is unconstitutional.

We don't hate this tax proposal. But we cannot avoid coming to the conclusion that, unfortunately, the banks are probably right.

The notion that the banks might be able to defeat the bank tax on constitutional grounds is risible, annoying, and enervating. But the world is often all those things. We can laugh, complain, and collapse with exhaustion but, as we used to say back when I was doing M&A, “it is what it is.”

And what the bank tax is is an unconstitutional Bill of Attainder.

The constitution bars bills of attainder in Section 9 of Article I: “No Bill of Attainder or ex post facto Law shall be passed.”

This was a truly revolutionary departure from English law, which had featured Bills of Attainder since the fourteenth century. But when the framers met in Philadelphia to write the constitution, they unanimously decided to ban them. Despite the contentiousness of much of the constitutional debate, there was not one objection raised to the ban.

Thanks to this legal innovation, most of us are not very familiar with attainder. Basically, it is a criminal conviction by the legislature rather than the judiciary, circumventing due process protections of the accused. It is closely linked with “ex post facto” law making, since often the person was punished despite not having violated any law at all.

There is not a lot of US legal precedent when it comes to bills of attainder. But what there is suggests that a bill of attainder might be said to have five elements:

Ex Post Facto: It penalizes actions committed before the law is passed.

It penalizes actions committed before the law is passed. Severe: The penalty is severe enough that it is readily identifiable as punitive.

The penalty is severe enough that it is readily identifiable as punitive. Punitive Intent: It was passed with the intent to punish.

It was passed with the intent to punish. No Legit Purpose: There is no legitimate, non-punitive purpose for the law.

There is no legitimate, non-punitive purpose for the law. Targeted: It applies to an identifiable individual or group.

It applies to an identifiable individual or group. Unavoidable: The targets cannot change their behavior to avoid the penalty.

The new bank tax pretty clearly qualifies under each of these elements.

Ex Post Facto: The tax attaches to banks that took TARP funds that were offered by the government to shore up the financial system. This was—and remains—perfectly legal. Yet now banks are being penalized for participating in the government program.

Severe: The tax is expected to confiscate hundreds of billions of dollars from the banks, which surely qualifies it as severe. Courts have found that to be the punitive confiscation of property.

Punitive Intent: Obama’s remarks introducing the tax proposal sounded very vindictive to almost everyone who has heard it. Joe Biden emailed Coakley supporters in Massachusetts that Scott Brown's opposition to the tax was "a perfect opportunity to show where his loyalties lie -- with working families in Massachusetts or with the bankers on Wall Street who helped lead us into the mess we're in -- guess who he chose?" That's the language of punishment for the financial crisis.

No Legit Purpose: There’s no other legitimate purpose to the tax but to punish banks for being bailed out. It will probably be argued that the tax will reduce risk at banks—but if that were anything more than a pretext the tax would be permanent rather than expiring when the TARP is fully repaid. And the government cannot argue that the purpose is the collection of revenue—that would be an absurdly broad rationale that would exempt any confiscation, fine or tax from the ban on Bills of Attainder.

Targeted: The tax is obviously targeted at the large banks that took TARP. No one expects the list of banks that qualify for the tax to grow, and Obama’s proposal actually exempts the automaker TARP recipients. The courts have ruled that corporations count as individuals who cannot be targeted in this way, so there’s no getting around it by saying Bank of America isn’t a person. And just because there are dozens of banks that will be taxed doesn’t mean it is too broad to qualify—the Supreme Court once ruled that a law aimed at the entire membership of the communist party counted as a Bill of Attainder.

Unavoidable: There’s nothing the banks can do to avoid the tax. No management changes or better risk controls will exempt them. There is literally no new regulation to comply with in order to avoid the penalty. They’re stuck with the taint of TARP.

In short, the bank tax looks an awful lot like it should qualify as a Bill of Attainder. This means that lawmakers conscientious of their constitutional obligations should refuse to allow the tax to become law.

The president, once he is made aware of the attainder issue, should not sign it into law. And the courts should strike it down.

Whether or not any of that will happen is another issue. Lawmakers seem eager to show the public they can stand up to the banking lobby, and a punitive tax is just the ticket for that. Obama hasn’t shown any signs that he is open to being persuaded by arguments about the constitutionality of his proposals. And successful court challenges on attainder grounds are rare.