U.S. Sen. John Cornyn's reelection bid received a big cash assist from national Republicans late in campaign

A super PAC supporting the reelection of U.S. Sen. John Cornyn, R-Texas, received a late infusion of big money ahead of the November election, including from the top super PAC aligned with Senate Republicans, according to a new post-election campaign finance filing.

Cornyn ultimately won by a decisive margin — 10 percentage points — but the 11th-hour rush of donations shows his allies were moving to keep up as his Democratic challenger, MJ Hegar, benefited from a flood of outside spending in the contest's closing days. Due to a loophole in campaign finance law, the super PAC did not have to report its post-Oct. 14 donors until after the election, concealing who was funding its activities in the race's final days.

The filing shows that the pro-Cornyn super PAC, Texans for a Conservative Majority, received $4.3 million from Oct. 15 through the election, most notably including $1.2 million from the Senate Leadership Fund. The group is closely tied to Senate Majority Leader Mitch McConnell, and it largely spends in the most competitive Senate races.

The pro-Cornyn super PAC also got $1 million from Houston beer distributor John Nau and $250,000 from Houston construction executive James D. Pitcock Jr. There were nine other donors who gave $100,000 each.

The group spent heavily during the period, dropping $6.1 million on advertising, all against Hegar. The $6.1 million was far more than the cash on hand it had as of Oct. 14, raising the likelihood at the time that it was raking in millions during the final two and a half weeks of the contest.

The scramble to back up Cornyn was understandable. Hegar blew past Cornyn in fundraising after winning her July primary runoff, and once early voting began, a coalition of Democratic outside groups dumped eight figures into the race, making for an uncertain finish even as polls continued to give Cornyn the lead.

As Election Day neared, speculation simmered about whether Cornyn would need a last-minute boost from the National Republican Senatorial Committee, the campaign arm of Senate Republicans, or the Senate Leadership Fund. The NRSC said it had "full confidence" in Cornyn at the time.

On Wednesday night, a Senate Leadership Fund spokesperson downplayed its donation to Texans for a Conservative Majority.

"Just padding our win-loss record," the spokesperson, Jack Pandol, tweeted. "Credit on the impressive win goes to Team [Cornyn]!"

Disclosure: John Nau and James D. Pitcock Jr. have been financial supporters of the Tribune, a nonprofit, nonpartisan news organization that is funded in part by donations from members, foundations and corporate sponsors. Financial supporters play no role in the Tribune's journalism. Find a complete list of them here.