Here we go again.

The scandal over bank manipulation of a key interest rate is just the latest strong signal that bankers rigged the system to benefit themselves and screw everybody else.

Not that we need another signal.

The scandal stems from something called LIBOR – the London Interbank Offered Rate. It’s an integral part of the global banking system. LIBOR is supposed to reflect the interest rate at which banks loan money to each other. It’s also a benchmark rate for other transactions, everything from home mortgages and credit cards to complex derivatives.

That means that the cost of the mortgage loan is pegged to whatever LIBOR is. On a home mortgage loan, for example, the interest rate might be a few points above LIBOR. The Financial Times estimates that about $350 trillion worth of contracts are tied to LIBOR.

It turns out that British-based Barclays Bank was manipulating the rates to increase their own profits, and to disguise how the bank was performing­ – possibly with the collusion of their regulators. The conservative Economist calls it “the rotten heart of finance,” and cautions that it is about to go worldwide.

The scandal hit home in England first, causing Barclays’ Bank president to resign and pay a record fine, and regulators on both sides of the Atlantic promising to get to the bottom of it.

But there are strong suspicions that Barclays wasn’t alone, that other too big to fail banks might have also engaged in the same shenanigans. The Wall Street Journal reports that at least 16 banks are under investigation, in three criminal and 10 civil probes.

It’s bad enough that Barclay traders have been caught discussing the manipulation in emails, referring to the rate manipulation as “the fixings” and requesting a particular rate as casually as if they were ordering a double latte.

What’s worse, the Financial Times started raising questions about the LIBOR-rigging five years ago and the Wall Street Journal cast doubt on the banks’ LIBOR practices in May 2008. 2008. So any regulator or prosecutor with an iota of curiosity could have been digging into LIBOR since then.

As we already know, curiosity about bankers’ malfeasance has been a rare commodity among the officials who are supposed to be scrutinizing their bank behavior. Remember President Obama’s repeated promises to get tough on bankers, most recently in his State of the Union speech in January?

Don’t expect Mitt Romney to make an issue of it – at least 15 of Barclay’s most senior U.S.-based bankers have donated the maximum $2,500 contribution to his presidential campaign. The CEO who resigned, Bob Diamond, had been among the co-hosts for a London fundraiser when Romney goes to London for the Olympics. (Barclays’ political action committee has also contributed significant amounts of cash to Democrats, though not the president, over the years.)

The LIBOR scandal rips the curtains away from one of the nastiest Big Lies on both sides of the 2012 presidential campaign: the president’s line that his Dodd-Frank reform has fixed the financial system, and Romney’s pitch that regulation is the problem and that we should leave bankers alone to run their business as they see fit.