Senate Majority Leader Harry M. Reid (D-Nev.) says a Bain Capital investor told him that Mitt Romney didn’t pay taxes for 10 years. (J. Scott Applewhite/AP)

Senate Majority Leader Harry M. Reid (D-Nev.) turned heads this week by suggesting that Mitt Romney is withholding his tax returns because he didn’t pay taxes over a 10-year stretch.

In doing so, Reid burnished his reputation for lobbing occasional stinging insults — and seized on a Democratic attack line that seems to be resonating with voters.

Reid said Tuesday in an interview with the Huffington Post that he had learned of Romney’s tax history from “a person who had invested with Bain Capital.” But when pressed, he declined to identify the investor or cite any proof for his allegation.

Reid spokesman Adam Jentleson declined to share more information about the investor, saying in an e-mail that “I have no information beyond what he said. If Mitt Romney is upset with Senator Reid’s remarks, there’s a simple way for him to clear up the matter: release his tax returns.”

The Romney campaign did not respond to requests for comment; it has previously denied rumors that the Republican presidential challenger hasn’t paid taxes in any given year. Romney himself has said that releasing more than two years of his tax returns would only provide fodder for President Obama’s reelection campaign.

But recent polls suggest that voters expect presidential candidates to be more forthcoming about their taxes: Just over half of voters in the swing states of Florida, Ohio and Pennsylvania think candidates should “publicly release several years of tax returns,” according to a poll released Wednesday by Quinnipiac University, the New York Times and CBS News. And a USA Today-Gallup poll published last month found that a majority of Americans — and almost a third of Republicans — think Romney should release more tax information.

Seizing the political momentum, Reid and other Senate Democrats are expected to continue pressing Romney on taxes when the debate on extending George W. Bush-era tax cuts begins after this month’s congressional recess. Aides said Reid may also make the point in his speech next month at the Democratic National Convention in Charlotte.

One senior aide defended Reid’s comments by noting that he has previously expressed concerns about Romney’s taxes in Senate floor speeches and weekly news conferences. “His own political instincts tell him this is a huge vulnerability for Romney, and his boxing instincts tell him to keep hitting his opponent in his weak spot,” said the aide, who spoke on the condition of anonymity to frankly discuss the senator’s intentions and who was referring to Reid’s former boxing career.

Whether or not the senator’s statement was true, it adds to a long list of remarks by the Democratic leader that Republicans call “foot-in-mouth moments.”

In 2004, Reid called Bush a “liar” in response to the president’s plan to move nuclear waste to Yucca Mountain in Nevada — a chief home-state concern of Reid’s. He called Bush a “loser” in 2005 during a meeting with high school students. He later apologized for the “loser” comment but never did so for calling the president a liar.

Several big life insurance stocks fell sharply in October 2008 when Reid hinted at a potential bankruptcy in the industry. Before the Senate approved a $700 billion bailout bill, Reid said that “a major insurance company — one with a name that everyone knows” — was on the verge of going broke. But a spokesman later said the senator was unaware of any such scenario.

In June, Reid dismissed questions about an immigration bill by responding, “That’s a clown question, bro.” He borrowed the phrase from Nevadan-turned-Washington Nationals superstar Bryce Harper, a rookie outfielder.

Beyond the halls of Congress, Reid called then-Federal Reserve Chairman Alan Greenspan “one of the biggest political hacks we have here in Washington” and said Supreme Court Justice Clarence Thomas was “an embarrassment” and ill-equipped to serve as chief justice. He even knocked members of the public in 2008 when he said, “You could literally smell the tourists coming into the Capitol.”