Puzder, the chief executive of Carpinteria-based CKE Restaurants Inc., the parent company of the Carl's Jr. and Hardee's chains, is scheduled to face tough queries at a Senate hearing Thursday.
Warren gave him a preview Monday in a 28-page letter.
"My staff's review of your 16-year tenure as CEO of CKE Restaurants, Inc….reveals that you've made your fortune by squeezing the very workers you'd be charged with protecting as Labor Secretary out of wages and benefits," Warren wrote.
"Your company's record of prolific labor law abuses and discrimination suits — the most of any major burger chain — gives me great pause given that as Labor Secretary you'd be charged with enforcing these very laws," she said.
Warren cited a January report by Capital & Main, a California news website, co-published with Newsweek that found that since Puzder became chief executive in 2000, CKE had faced more federal employment discrimination lawsuits per billion dollars of sales than any other U.S. hamburger chain.
Puzder is the last of the eight nominees targeted by Senate Democratic leaders to have a hearing after scheduled sessions were delayed four times as the committee awaited his ethics and financial disclosure paperwork.
The Senate Health, Education, Labor and Pensions Committee received Puzder's paperwork last week. In it, he promised to divest his multimillion-dollar stake in CKE Restaurants and sell a wide array of other investments.
Puzder also promised not to participate in any matters as Labor secretary involving CKE Restaurants unless he received a waiver or authorization in conjunction with federal law.
The other seven nominees targeted by Senate Democratic leaders have either been confirmed or are on track for confirmation this week. One of them, Treasury secretary pick Steve Mnuchin, was confirmed by the Senate on Monday night.
Senate Democratic leaders are now focusing on Puzder, believing his nomination is vulnerable because of his long tenure heading a fast-food chain and his admission last week that he had employed a housekeeper for years who was in the U.S. illegally.
On Thursday, Senate Minority Leader Charles E. Schumer (D-N.Y.) called for Puzder to withdraw, calling him "probably the most anti-worker" choice ever for the Cabinet position.
At least three Republicans on the committee have yet to commit to voting for Puzder. If they decide to oppose him, and if all Democrats vote against him, that would be enough to derail his nomination.
Warren tried to keep up the pressure with her Monday letter. She told Puzder his record of public comments over the years "reveals a sneering contempt for the workers in your stores and a vehement opposition to the laws you will be charged with enforcing."
She fired off questions about his potential conflicts of interest, labor law violations at his company's restaurants and his position on various Labor Department rules and investigations. She asked for answers no later than Feb. 21.
Democrats have been critical of labor law violations at CKE Restaurants, and fast-food workers have staged protests against Puzder's nomination in Los Angeles and elsewhere. Workers in the Fight for $15 movement rallied against Puzder Monday at CKE's Anaheim offices and in several other cities.
Democrats and worker rights advocates also have criticized Puzder for his opposition to increasing the federal minimum wage to $15 an hour and expanded eligibility for overtime.
An analysis by Bloomberg BNA found that about 60% of Labor Department investigations of Carl's Jr. and Hardee's restaurants since 2009 turned up at least one violation of the Fair Labor Standards Act, which covers minimum wage, overtime and other regulations.
But Puzder's backers have noted that was one of the best performances by leading fast-food outlets.
Business groups have been trying to rally support for Puzder.
On Monday, more than 100 trade associations sent a letter to senators urging them to confirm Puzder.
"Mr. Puzder has seen, firsthand, the impact of burdensome regulations promulgated by the previous administration, and he has seen both the negative impacts and unintended consequences they can have on employers and employees alike," the groups wrote.
"Being a respected, proven leader in an industry that represents approximately 10% of the workforce gives him a unique set of qualifications to serve in this position," they said.
In addition, supporters are focusing on Republicans who have yet to commit to voting for Puzder.
On Friday, for example, the Alaska Cabaret, Hotel, Restaurant and Retailers Assn. wrote to Sen.
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2:45 p.m.: This article was updated with details about Sen. Elizabeth Warren's charge about CKE discrimination lawsuits and confirming that protests against Puzder took place in Anaheim and other cities.