A mysterious Texas-based super PAC that received $2.3 million from undisclosed donors to run last-minute ads in support of Democratic Rep. Beto O’Rourke was funded by the Senate Democrat-aligned Senate Majority PAC, according to new federal election filings made public Thursday evening.

SMP donated the money to the super PAC, Texas Forever, in a way that circumvented federal election deadlines that trigger donor disclosure — an increasingly common tactic employed by both Democrats and Republicans that, while legal, critics say violate the spirit of disclosure requirements.

The O’Rourke campaign raised another $10 million in donations after mid-October — bringing his total fundraising for his failed Senate bid to a record-high $80.5 million and making him one of the most successful fundraisers in American politics. Half of the $10 million came from donations under $200.

O’Rourke’s campaign spent nearly all of that money, with less than $480,000 in cash on hand after his campaign ended, new records show.

The flurry of last-minute activity to support O’Rourke in the Senate race reflects the national attention and deep fundraising base enjoyed by the 46-year-old congressman, who consistently polled behind the incumbent and ultimate winner of the race, Sen. Ted Cruz (R). Still, O’Rourke is now weighing a 2020 presidential bid.

Texas Forever is one of more than a dozen super PACs that emerged last-minute or ran ads for or against campaigns without disclosing their donors until a month after the election, new filings show. Some of these super PACs were tied to well-known national groups, but others were funded by just a handful of wealthy donors.

Michael Toner, former Republican chairman of the Federal Election Commission, said super PACs from both parties are taking advantage of a “critical hole in the disclosure regime” for super PACs.

“The general rule is that contributions received by super PACs and expenditures made by super PACs are publicly disclosed, and I think there’s broad consensus on the appropriateness of that,” Toner said. “The value of disclosure after Election Day is not nearly as pertinent as it is before Election Day.”

Campaign Legal Center, which advocates for stricter regulation of money in politics, estimated in a new report that such super PACs spent upward of $29 million in the final weeks of the 2018 election without disclosing their donors or reporting the full extent of their spending before Election Day.

Many of the groups appear to have local ties, with a mailing address based in the district or the state, and a name that suggests local donors launched or funded the group, said Brendan Fischer, who directs the federal regulatory work of the Campaign Legal Center.

Yet many of these groups that were active in the primaries were revealed after the primary election to be tied to national, party-connected super PACs, meaning they may have given a misleading impression to voters before they cast their ballots, Fischer said.

“The tactics that we’ve seen super PACs use widely this election cycle undermine the transparency that voters should have a right to expect from the groups spending millions of dollars trying to influence their vote,” Fischer said.

This was not the first time SMP, which is aligned with Senate Minority Leader Charles E. Schumer (D-N.Y.) and works to elect Democrats to the Senate, was the primary funder of a “pop-up” super PAC.

It was the primary donor to Red and Gold, a super PAC that was active in the Arizona Senate primaries attacking Republicans. It was the main giver to Highway 31, contributing $3.1 million of the $4.5 million the super PAC spent to help Democrat Doug Jones win the Alabama Senate race in 2017. In both cases, the super PACs were structured so that donors would not be disclosed until after the election.

SMP declined to comment.

In Ohio, a new super PAC named MeToo Ohio spent hundreds of thousands of dollars on independent political ads attacking Sen. Sherrod Brown (D). In Thursday’s filings, the group disclosed that its major funder was another super PAC funded by a handful of wealthy Republican donors and a politically active nonprofit organization that does not disclose its donors.

DefendArizona, a super PAC active in the Senate race in Arizona, disclosed about $300,000 in cash on hand on Oct. 17, about three weeks before the Nov. 6 election. Then, on Thursday, the FEC reports showed it spent $7.7 million on independent political campaigns in those final weeks in support of Rep. Martha McSally (R).

Of that amount, $1.9 million came from Senate Leadership Fund, the super PAC aligned with Senate Republican leadership, the Thursday FEC reports show. Ken Griffin, a major Illinois-based GOP donor and chief executive of hedge fund Citadel, gave $2 million.

The last public filings before Election Day were due Oct. 17. The filings made public Thursday evening cover donations and expenditures from Oct. 18 through Nov. 26.

The new filings also showed the last-minute rush among donors to inject millions into established super PACs supporting congressional candidates from both parties.

America First Action, the super PAC supporting President Trump’s agenda, brought in $4.8 million between Oct. 18 and Nov. 26. Don McGill Toyota, a Toyota dealership in Katy, Tex., gave $1 million of that sum. Another $1 million came from Darwin Deason, a major Texas GOP donor and chief executive of Deason Capital Services.

Senate Democrat-aligned Senate Majority PAC received $14.1 million during the period, with $1.1 million from Majority Forward, a nonprofit group that does not disclose its donors, and $1 million from heavyweight Democratic donor George Soros.

The Senate Leadership Fund, aligned with Senate Majority Leader Mitch McConnell (R-Ky.), reported raising $9.1 million, with $5.7 million coming from its sister nonprofit, One Nation.

The House Majority PAC, the Democratic super PAC supporting House candidates, saw a last-minute funding of $19.2 million, with the most money — $4 million — coming from billionaire hedge fund manager James Simons.

The Congressional Leadership Fund, the House GOP-aligned super PAC, reported $14 million, with $5 million coming from billionaire Republican donors Sheldon and Miriam Adelson, bringing their total political giving in the 2018 election cycle to $117 million. Another $4.5 million came from the American Action Network, a politically active nonprofit group that is connected to the Congressional Leadership Fund.