The Senate Republicans are refusing to allow a vote on the Merkley-Levin amendment, which would put a meaningful version of the Volcker Rule into law (splitting off proprietary trading from major banks).

After weeks of dancing around, the Democrats finally have a signature issue on which to fight. Senator Carl Levin frames it exactly right: “It’s a sad day when the power of Wall Street can overwhelm the power of the American people in the US Senate.”

This is the opportunity that White House claims it has long sought – to have an intense fight on a financial reform issue that everyone can understand. Paul Volcker made his determination long ago: the big banks are too big and must, in this fashion, be broken up. Senators Merkley and Levin negotiated the precise language of their amendment in good faith. The Republicans have made their answer clear: No way.

Time for President Obama to make the call.

Only the president can break through the daily logjam of information. Only the president can define the issues in the simple, powerful and convincing terms that people can grasp. Only the president can insist – this is a matter of urgent national priority.

The economic analysis (Volcker), political back-story (Brown-Kaufman and all that involved), and just the right rhetoric are already in place:

“We got into this financial crisis because Wall Street set the rules to benefit itself, and now with an assist from Senate Republicans, they’re doing it again,” said Merkley. “Obviously the lobbyists are afraid they’ll lose this vote, and in typical Wall Street fashion their solution, with help from Senate Republicans, is to rig the result. Main Street is being shut out of this debate. It is time to stop letting Wall Street call the shots – let this amendment have a vote.” “The long arm of Wall Street reached directly into the Senate chamber today,” Levin said. “By blocking us from even debating this amendment, the Republican leadership is carrying Wall Street’s water and standing in the way of real reform.”

This is a defining issue for the president. Either he takes up the Volcker Rule – proposed by his administration, to great fanfare (and some skepticism) in January. Or he rolls over – admitting that Wall Street has won.

We know where Goldman Sachs and its fellow travellers stand on this issue – adamantly and publicly opposed. And we pointed out here in February which way the Republicans were likely to go.

“But if you don’t have the votes in the Senate, what can you do?” This one is easy. You stop the clock and put everything else on hold. The president calls the American people to order and asks them to take a long hard look at the issues and the corporate interests at stake.

And then you start to pound away. Day in and day out, the president and other leading members of his administration need to come out swinging with relentless pursuit of substance on TV talk shows and prime time speeches – demanding an up-or-down vote on Merkley-Levin.

Admittedly, this may be awkward for leading officials, who have been rather accommodative to financial interests over the past 15 months or so. That’s unfortunate (for them), but now entirely water under the bridge. All is forgiven to the policymaker who finally gets it and changes course in the right direction.

Don’t move on. Pick up the baseball bat that Paul Volcker has given you. Either that or go down to the most embarrassing, humiliating, and memorable defeat in the history of Wall Street-Washington confrontations. It’s the president’s call.