The Levin brothers are lashing out at a fellow son of Michigan, drawing attention to Mitt Romney’s tax returns, which have become a favorite Democratic talking point against the presumptive GOP presidential nominee. In the process, the two Michigan Democrats are highlighting their long crusade against offshore tax havens.

The pair want to amend the Ethics in Government Act of 1978 to require presidential candidates to release 10 years’ worth of tax returns and fully disclose all assets and bank accounts.

House Ways and Means ranking member Sander Levin said Romney should release a decade of tax returns and information about his offshore bank accounts.

“Gov. Romney needs to immediately release his tax returns and also explain [them] so they’re understandable to the American people, to the typical family,” he said. “Gov. Romney should set the example and release his returns. More and more, there is the understanding that he should do so.”

The information is particularly important, he said, because Congress might consider overhauling the tax code next year.

“I think we should have some understanding about what has been the past practices of candidates,” he said.

Speaker John Boehner and Republicans leaders, though, have little patience for talk of Romney’s tax returns. When asked about the issue at a Wednesday press conference, Boehner returned to his favored talking points on the economy. “It is not about the tax returns. It’s about the economy,” he said.

“The American people are asking, ‘Where are the jobs?’ They’re not asking where in the hell the tax returns are,” the Ohio Republican said. “This is another sideshow intended to draw the American people’s attention away from the real issue, and the real issue is that the president’s economic policies have failed.”

Arizona Sen. John McCain, the GOP standard-bearer in 2008, made similar comments Tuesday.

House Budget Chairman Paul Ryan, a Romney surrogate who is also in the running to be the candidate’s vice presidential choice, said he’d defer to the campaign on the issue.

“These side issues are merely distractions from the task at hand, which is that the president is not leading, and as a consequence of that, Americans are hurting,” the Wisconsin Republican said.

The comments do not bode well for the legislation’s chances of coming to the House floor, as Republicans have little incentive to embarrass their candidate before the elections.

Democratic Sen. Carl Levin (Mich.), Sander Levin’s younger brother, has been criticizing the abuse of offshore tax accounts long before the current presidential election cycle.

Sen. Levin is chairman of the Senate Homeland Security and Governmental Affairs Permanent Subcommittee on Investigations.

“We’ve had hearings on this throughout the decades,” he said. “We’ve gone through schemes, after schemes, after schemes when offshore tax havens were used to avoid paying taxes.”

Earlier this year, he attached a provision to the Senate’s highway bill that would have given the Justice Department authority to combat tax avoidance. He blasted House GOP negotiators for stripping the language in the House-Senate transportation conference.

“That vote by the House allows the wealthy and powerful to continue dodging the taxes they owe, increasing the tax burden on American families who abide by the law and their tax obligations,” he said on the Senate floor Wednesday.

The Levin brothers are not alone. Senior Senate Democrats have embraced the attacks on Romney over his position on tax returns and use of offshore tax havens.

Majority Leader Harry Reid (D-Nev.) has repeatedly cited Romney’s tax records during press conferences and speeches on the Senate floor, and his top leadership lieutenants have joined in the messaging campaign.

For instance, Majority Whip Dick Durbin joined Sen. Levin at a pen-and-pad briefing with reporters Wednesday to promote a bill that would require Members of Congress and presidential candidates to disclose holdings in offshore tax shelters.

The Illinois Democrat was not shy in discussing the motivation for the renewed push for the bill, which he introduced in March with fellow Democratic Sen. Al Franken (Minn.).

“My research suggests that Mitt Romney is the first presidential candidate in American history with a Swiss bank account,” Durbin said.

“It raises questions. Why does he not want to provide the same information his father provided? George Romney set the standard: 12 years of income tax returns when he ran for president. That was the standard for presidential candidates,” he added. “His son has produced one year and said he may produce one more.”

Sen. Levin said Durbin’s disclosure bill has obvious merit.

“Anything which can bring to light and make transparent what has been going on is valuable, as far as I’m concerned,” he said.

Durbin said he would try to attach his proposal to the next moving vehicle on the Senate floor that allows nongermane amendments.

For his part, Rep. Levin said in a Wednesday conference call that he hopes at least some Republicans sign on to the bill and that it can become a long-term push to increase disclosure among candidates for public office.

“I think the more people think about this, the more they realize that there is a responsibility for people who want to become perhaps the most important leader in the entire world,” he said.