The so-called “Volcker rule” lost a key potential supporter yesterday, after a top Republican senator who proponents had hoped would back the plan to ban proprietary trading at banks questioned the practice’s role in the financial crisis.

Many had hoped Alabama Sen. Richard Shelby would support the plan, named after ex-Federal Reserve chairman Paul Volcker and designed to limit bank risk by, among other things, barring banks from trading securities for their own benefit. However, in the month since the White House introduced the proposal, Shelby has kept his distance.

“Sen. Shelby supports the spirit of the Volcker rule — limiting risk to financial institutions and the taxpayer through excessive risk taking — but is not convinced that the proposal itself is necessary,” Shelby’s office said in a statement.

The Post reported Tuesday that the Obama administration is backing off an outright ban on prop trading, favoring instead an alternative in which federally insured banks that engage in prop trading would be required to put up cash reserves to protect against losses.