 -- The White House declined to criticize Democratic Senate Majority Leader Harry Reid over his unsubstantiated charge that Mitt Romney may not have paid taxes at all for a decade.

"I would simply say that you all probably know Sen. Reid well and, you know, he speaks for himself," press secretary Jay Carney told reporters at his daily briefing.

Romney has denied the allegation, and top Republicans have branded Reid a liar —but some Democrats have said the former Massachusetts governor could banish any doubts by releasing his income tax returns for that period, something he has steadfastly refused to do.

"I think it is a fair point to make that this is an issue that was not originated during the general election campaign, did not start with the president's campaign or with Sen. Reid. It started in the Republican primary when Gov. Romney's opponents brought up this issue," Carney said.

"The word's out that he hasn't paid any taxes for 10 years," Reid said of Romney on the Senate floor Thursday. "Let him prove that he has paid taxes, because he hasn't." The lawmaker was repeating the charge he made in a floor speech in July and again more recently in the Huffington Post.

Even as he seemed to try to distance the White House from the escalating feud, Carney pointedly noted that it was Romney's father, George Romney, who set the modern standard for presidential candidates in 1968 by releasing a decade's worth of income tax returns.

"You know, it's not always every candidate's favorite part of the process, but it's a tradition that's important; it's valuable to the American people as they decide who should be president," said Carney. "So that's why the president has put forward his financial information, his tax returns when he was a candidate."

With the sour economy weighing down Obama's re-election prospects, the Democrat's campaign has argued that Romney cannot be trusted to fight for the middle class—using the Republican candidate's personal finances against him.

"The reason why this is an issue at a policy level is the president believes very strongly—and this is central to the debate we're having in this country—that we need to have greater tax fairness," Carney said.

That means ending tax policies "that allow a Warren Buffett or a hedge fund manager or other very affluent Americans to pay taxes at a much lower rate than a factory worker on a—at a plant, a GM plant in Michigan or Ohio," Carney said.

"I mean, you know, it's just about making our tax code more fair and making our tax code more helpful to the middle class, which is the backbone of our country and backbone of economic security and economic growth in our country," he said.