As Congress cracks down on lobbyist loopholes, it's hard to imagine a single dollar of a political contribution could still slip into the system anonymously.

Yet hundreds of thousands do, says the political money watchdog Center for Responsive Politics, through what it says is a loophole in need of closing.

Under current rules, House and Senate lawmakers can create a particular type of campaign finance organization to take in money from lobbyists, special interests and other major donors -- up to $5,000 per contributor per year -- and spread that money to the campaign coffers of other lawmakers.

They are known as leadership political action committees. And unlike other money-raising organizations, they don't have to tell the public which lawmaker controls them, allowing many to obscure the path of potentially influential cash into federal politics.

Candidates "raise money in Washington for their leadership PAC, and they raise money at home for their campaign," campaign finance expert Brett Kappel told the Blotter on ABCNews.com. "Many would much prefer their constituents at home never know they had a leadership PAC." Much of the money raised for leadership PACs comes from lobbyists, Kappel said.

Click Here for Full Blotter Coverage.

By giving chunks of that money to their fellow House and Senate members, "ambitious lawmakers can gain clout among their colleagues and boost their bids for leadership posts or committee chairmanships," CRP notes on its Web site, OpenSecrets.org. They can also "hire additional staff -- sometimes even family members," with the money, the group said.

Such was the case with likely GOP presidential hopeful and ex-Sen. Fred Thompson, whose leadership PAC paid out nearly $180,000 to his son Daniel before it was shut down earlier this year.

Thompson made no secret that he controlled the group. He named it the "Fred D. Thompson PAC." 

But others don't. In the 2006 election cycle, 35 anonymous leadership PACs raised more than $1.2 million, without a single mention in a press release or public filing of the lawmaker which controlled them.

If Congress approves a proposed bill by Rep. Walter B. Jones, R-N.C., the groups would be required to report which lawmaker controls them.

Until then, CRP has enlisted the public's help in tracking down the lawmakers which control the mystery organizations, in what they've dubbed their "Mystery PAC" effort.

So far, private citizens, reporters and other government watchdog groups have helped identify the lawmakers behind 10 of the anonymous leadership PACs, CRP reports on its Web site.

"This has already been a success as far as we're concerned," said CRP president Sheila Krumholz of the group's effort.

Do you have a tip for Brian Ross and the Investigative Team?