Mitt Romney gets plenty of grief for refusing to release more of his tax returns, but his potential running mates are proving just as tight-fisted.

Of the five VP picks considered most likely, Sen. Marco Rubio is the only one who has disclosed multiple tax returns. During his 2010 Senate run, Rubio released nine years of data from 2000 to 2009. But the move was short-lived; the pages can no longer be found on his campaign website and now only exist in the annals of newspaper articles and blog posts.

Bobby Jindal released his 2006 tax return in 2007, during his successful run for Louisiana governor. The others believed to be on Romney’s so-called short list—Sen. Rob Portman, Rep. Paul Ryan, and former Minnesota governor Tim Pawlenty—have all refused to go public with their tax returns. Their offices either declined to comment or did not respond to requests.

Members of the Senate and the House have a long-standing aversion to making their returns public. Although members of Congress are required to release certain financial documents each year, federal tax returns are not covered by the law. And unlike presidential candidates, who have a long history of releasing tax returns—a practice that became consistent starting in the early 1970s—lawmakers don’t adhere to the same tradition.

In a recent poll by National Journal, every Republican and nearly two-thirds of Democrats in Congress who were surveyed said that members should not have to release any tax returns.

“Members of Congress already have little financial privacy and spend a disproportionate amount of time trying to comply with vague and voluminous ethics requirements,” wrote one Republican anonymously. “All this would do is increase the amount of [opposition] research in the world, not a step forward for democracy.”

When House Minority Leader Nancy Pelosi was asked about her returns, she told reporters, “When I run for president of the United States, you can hold me to that standard.”

Alex Patton, a Republican strategist, says the reason for keeping the documents secret is obvious: politicians don’t want to give their competition any ammunition.

“Most of them are very wealthy,” he says. “If you have a very simple tax return and a W-2 that says you are making $50,000 a year, then that’s one thing, but if you have one that says you are making dividends and the like, you just open yourself up to a bunch of potshots.”

Patton says it won’t matter whether Romney chooses a candidate with a better record of exposure or one who is more secretive because “any VP pick will release tax forms.”

But Democratic consultant Christopher Crotty says he believes that whether a potential running mate has released tax returns will loom large in Romney’s decision. He says Romney will want a running mate whose handling of the issue won’t make the former governor look bad.

“I believe that would be the campaign’s preference and Romney would continue to withhold his tax returns,” says Crotty. A candidate who has released more returns, he says, “would make Romney look hypocritical.”