Most lawmakers, however, chose to keep their tax liabilities a secret.

“First your publishers and editors and execs should publish their tax returns. They have great influence over public policy,” Rep. Gary Ackerman, D-N.Y., said in an emailed response. Ackerman, who was dogged in 2010 by allegations of a sweetheart stock purchase, isn’t seeking re-election, so his term ends in January.

“Are you guys asking the president to turn over his college records? Or asking him to turn anything over of any kind?” responded Allen Klump, the communications director for Rep. Jeff Duncan, R-S.C.

“Thanks, but we will not give you Sen. Rockefeller’s tax return. Good luck on the project,” said Vincent Morris, a spokesman for Sen. Jay Rockefeller, D-W.Va., whose family surname is synonymous with wealth.

Several lawmakers said they’d disclose what’s required and no more.

Rep. Renee Ellmers “files a financial disclosure form each year in accordance with House ethics rules and this is publicly available,” said Tom Doheny, a spokesman for the North Carolina Republican.

Rep. Joe Wilson “has submitted a financial disclosure form, which is required by law and available to constituents as a matter of public record,” said Caroline Delleney, a spokeswoman for the South Carolina Republican.

Constituents generally know where their particular lawmakers stand on the issue of the expiring tax cuts because the two major political parties have well-defined views. But there’s more at stake than just tax brackets, and voters often have little feel for whether their members of Congress would benefit or be harmed by changes that are under consideration.

From the financial disclosure forms, constituents can, with some work, surmise how lawmakers’ investment income might be taxed under competing plans. Given their salaries, lawmakers would fall into the higher tax bracket for dividends, but it’s unclear where they’d fall individually on the income scale.

“There are clearly some people above (the $250,000 threshold). There’s a bunch of people who might or might not be affected, and you can’t tell,” said Roberton Williams, a senior fellow at the Tax Policy Center, which is run by the Brookings Institution and the centrist Urban Institute.

Tax data isn’t always a panacea, however.

Missouri Democrat McCaskill was one of the few senators who provided McClatchy with a tax return. Her 1040 form lists her as married filing separately, showing an adjusted gross income of $193,384.

But her husband, Joe Shepard, is a wealthy businessman whose investments sometimes have put her in an unpleasant spotlight. His investment in a reinsurance company in Bermuda – the same country in which a Romney investment has been criticized by Democrats – brought allegations from Republicans in 2009 of tax dodging. Shepard no longer hold the investment.

McCaskill does report dozens of her husband's investments in her annual financial disclosure statement, with more detail than required. But each still is listed only under ranges of values, not precise amounts.

That’s another reason advocacy groups think that financial-disclosure reporting should be expanded to capture spousal income more fully, and argue that tax data would be a useful, albeit imperfect, tool.

“As public officials, potential conflicts of interest caused by their wealth and assets are a public concern,” said Holman, the Public Citizen lobbyist.

Kaz Komolafe and Farah Mohamed contributed to this article.