Pelosi argued that for presidential candidates, the standard is different. "There has been a tradition that tax returns are released," she said. 

Pelosi tried to turn the tables on her questioners at one point, saying: "Some people think the same standard should be held for the ownership of the news media ... who are writing these stories." 

Romney so far has agreed to release two years' worth of returns, but no more. He has suggested he doesn't want to give Democrats fodder for their opposition research. 

Democrats, though, have used Romney's reluctance to release more tax records to paint him as secretive. 

House Speaker John Boehner came to Romney's defense Thursday, calling the whole debate a "sideshow." He argued that neither presidential candidates nor members of Congress should have to release their records. 

"I've never released my tax returns. That's my private business just like it's your own private business," he said.