Billionaire casino owner Sheldon Adelson gave $1 million to a super PAC active in Michigan’s U.S. Senate race during the campaign’s final days, a fact unknown to voters until long after polls closed.

Adelson supplied the bulk of funding for the “Hardworking Americans Committee” with the Oct. 19 donation, Federal Election Commission records show.

The super PAC spent more than $1 million on ads in a futile, last-minute attempt to boost former Republican Rep. Pete Hoekstra in his bid to oust incumbent Sen. Debbie Stabenow, a Democrat.

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Liberal super PACs spent little – just $1,700 -- attacking Hoekstra,  according to the Center for Responsive Politics.

The deadline for reporting donations made since Oct. 17 was Thursday.

Last-minute contributions are not unusual in politics, but thanks to the 2010 U.S. Supreme Court’s Citizens United decision and a lower court ruling, the amount a donor can give to outside groups’ electoral efforts is unlimited. Furthermore, donations to political action committees during the final three weeks of the election need not be reported until December.

The reporting gap should be closed, say watchdogs.

“Congress should amend our disclosure laws to give voters the information they need to make informed decisions on Election Day,” said Paul S. Ryan, an attorney at the Campaign Legal Center. “With current technology, disclosure is easier than ever for super PACs and other political players.”

Adelson, the top donor to super PACs in the 2012 election by a large margin, along with wife Miriam, also provided all $2 million of Republican Jewish Coalition Victory Fund’s war chest. The super PAC, which did not report any receipts before Election Day, pumped more than $1.7 million into advertising opposing President Barack Obama.

The Republican Jewish Coalition is a lobbying organization that seeks to “foster and enhance ties between the American Jewish community and Republican decision makers,” according to its website. It was started in 1985 and Adelson serves on the group’s board of directors

Similarly, “Freedom Fund North America,” a GOP-aligned super PAC established on Oct. 15, spent $990,000 in the final weeks of the 2012 race, mostly attacking incumbent Sen. Jon Tester, D-Mont., and former North Dakota Attorney General Heidi Heitkamp.

Both Democrats prevailed in their hotly contested races.

Adelson did not return a call seeking comment.

The entirety of Freedom Fund's $1 million budget came from Texas businessman and Republican mega-donor Bob Perry, according to FEC reports.

New records further show that billionaire New York City Mayor Michael Bloomberg provided nearly $10 million to the “Independence USA PAC,” whose priorities include gun control, marriage equality for same-sex couples and education issues. It was launched on Oct. 18 and reported spending $8.2 million on five House races.

Two of its favored candidates — Democrat Gloria McLeod of California and Democrat Dan Maffei of California — won.

Super PACs can accept donations of unlimited amounts from corporations, unions and individuals.

FEC Vice Chairwoman Ellen Weintraub, a Democrat, stressed the importance of people having such information before they cast their votes.

“I always think the public benefits from and is entitled to transparency about the sources of political funds,” she said. “As the Supreme Court said in Citizens United, ‘The public has an interest in knowing who is speaking about a candidate shortly before an election.’”

Republican attorney Brad Smith, a former FEC chairman who founded the Center for Competitive Politics, however, argues that the lack of disclosure of last-minute super PAC donations is not a "real catastrophe or something that is harmful to the election process."

The government's interest in “preventing corruption or its appearance” is served just as well "if information is released afterward," he said.

The only other large contributions that went to the anti-Stabenow super PAC came from Amway President Doug DeVos and Michael Jandernoa, former president and CEO of pharmaceutical company Perrigo, who each donated $100,000 to the group in October.

Ads from the super PAC accused her of dodging taxes on her “ritzy Washington, D.C., home,” voting for tax increases and “failing Michigan for years.”

Despite the spending, Stabenow easily captured nearly 60 percent of the vote. Yet the last-minute deluge earned ire from her campaign.

“The fact that secret money can be dumped into races like this, with no one knowing where the money came from until a month after the election, is awful for our democracy,”  said Stabenow spokesman Cullen Schwarz.

Hoekstra did not return a call seeking comment about the super PAC spending on his behalf.

The Center for Public integrity is a non-profit, independent investigative news outlet.  To read more of its stories go to publicintegrity.org.

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The financial industry is the biggest single contributor to a super PAC supporting Republican presidential front-runner Mitt Romney, accounting for 48 percent of the $43.2 million raised so far by the organization, according an analysis by the Center for Public Integrity.

Private equity funds and hedge funds were the biggest contributors to Restore Our Future, the pro-Romney political action committee, kicking in at least $13.5 million of the $20.5 million given by the financial industry, according to the CPI analysis of Federal Election Commission records. Most of the rest came from investment banks and other asset managers and so-called “non-bank lenders,” it said.

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The financial industry support is potentially significant because Romney, a former private equity executive, has said he favors repeal of the Dodd-Frank Wall Street Reform and Consumer Protection Act, passed in the wake of the mortgage meltdown to regulate the financial industry and protect consumers. He also has declined to be pinned down on whether he would favor eliminating the “carried interest” tax loophole, which has helped make private equity and hedge fund managers enormously wealthy over the years.

Click here to read the full CPI report.

NBC’s Michael Isikoff also has reported on the contributors to Restore Our Future, which is by far the best-funded of the super PACs backing presidential candidates in the 2012 election. Among the biggest is Texas homebuilder Bob Perry, who gave the group $3 million.