CHARLOTTE, N.C.—Senate Majority Leader Harry Reid lambasted Mitt Romney for refusing to release several years of tax returns, suggesting that no other presidential candidate in modern history has been as secretive.

"Never in modern American history has a presidential candidate tried so hard to hide himself," Reid said in his speech at the Democratic National Convention.

The Nevada senator said Romney has asked the country to "take his word" that there are no damaging revelations in his tax returns.

"Take his word? His word?" Reid said sarcastically. "Trust comes from transparency and Mitt Romney comes up short on both."

Reid's attacks came just weeks after he took to the Senate floor and accused Romney of not having paid taxes at all for the last decade. The Democratic leader cited a source close to Romney whom he wouldn't identify. Romney has denied Reid's accusations. Reid sought to contrast Romney with President Barack Obama, telling the crowd, "You know him." He praised Obama for bringing "courage and character" to the presidency and for doing the "right thing" even in the face of GOP opposition.

In a dig at Romney, Reid praised Obama for bailing out the Detroit auto industry—a move Romney opposed. And Reid claimed "some said he shouldn't move heaven and earth" to find and kill Osama bin Laden. But Obama did, he said.

He cast Obama as a president hobbled by Republicans—whom he described as "naysayers" who embrace "couldn'ts and shouldn'ts."

The Republican Party, Reid said, has become the party of the "wouldn'ts and won'ts," doing anything to block Obama instead of working together with Democrats on behalf of the country.

"If they won't stand up to Rush Limbaugh or Grover Norquist, what would make anyone think they would stand up for you?" Reid said.