Labor unions have donated more than $10 million to the Democratic senators who will question Labor secretary nominee Andy Puzder on Thursday.

The 11 Democrats on the Senate Committee on Health, Education, Labor, and Pensions have been allies of organized labor throughout their careers. Four of the leading Democrats have cashed in on more than $1 million for their campaign war chests, according to the Center for Responsive Politics.

Donation totals ranged from $32,000 for freshman Sen. Maggie Hassan (D., N.H.) to $2.1 million for Ranking Democrat Patty Murray (D., Wash.). Socialist Sen. Bernie (I., VT) had the second-highest haul, with $1.88 million in contributions from labor groups over the course of his Senate career. He also received substantial support for his insurgent presidential campaign against Secretary of State Hillary Clinton in 2016. He received about $243,000 from that sector during the drawn out campaign.

Puzder, the CEO of CKE Restaurants, is the first career businessman nominated to lead the Department of Labor since 1981. He has faced mounting criticism from labor unions and activist groups over his past opposition to minimum wage hikes and his pro-business philosophy. Democratic Minority Leader Sen. Chuck Schumer (D., NY) has vowed that his party will present a united front against Trump's nominees moving forward and specifically targeted Puzder at a press conference on Thursday, saying he planned on "using everything we can to stop" Puzder's confirmation.

"[Trump] campaigned as a populist. He's governing as a hard-right militant. That's what Puzder is," he said. "We hope Puzder will be withdrawn."

Puzder is considered to be Trump's most vulnerable nominee. At least four Republicans, including three on the HELP committee, are on the fence about the vote, according to recent reports. If just three of those members defect, Sen. Majority Leader Mitch McConnell (R., Ky.) would not have the votes to confirm Puzder.

The nomination has also resulted in a flood of unfair labor practice complaints against CKE, the parent company of fast food franchises Hardees and Carl's Jr. At least 33 complaints have been logged against the company in the past two months—though the company is nearly 100 percent franchised and small businessmen set schedules, work rules, and payroll.

Labor watchdogs said that Democratic senators' opposition is influenced by the millions of dollars that unions give the party and its candidates each election cycle. Matthew Haller, spokesman for the pro-Puzder International Franchise Association, said that committee Democrats are looking to serve their special interest supporters at the expense of American workers.

"This is perhaps the greatest example of a quid pro quo that’s ever existed in modern politics," he said. "Despite the continual decline of private sector unionization, politicians continue to double down with the same failed rhetoric and policies that the very union members these organizations purport to represent are rejecting at the grassroots level."

Patrick Semmens, spokesman for the National Right to Work Committee, said the big dollar donations are no accident because the the committee's work attracts the most ideological pro-union partisans.

"Inevitably, the most partisan pro-forced unionism senators seek to be on the HELP Committee, where they see it as their job to shield unions from government oversight and put their thumb on the scale for union bosses," he said in an email.

Puzder is scheduled to appear before the HELP committee on Thursday morning.