With Monday night's confirmation of Steven Mnuchin to head President Donald Trump's Treasury Department, outstanding leadership roles tasked with guiding the administration's economic agenda are beginning to dwindle.

Trump's economic council and business advisory teams already hitting the ground running with meetings, and the gang tasked with overseeing what the president has described as ambitious job creation and economic stimulation efforts is almost fully assembled.

Yet Trump's pick to fill one of the most key economic-related Cabinet positions appears potentially in peril. According to a Washington Post report, four GOP senators – Susan Collins of Maine, Johnny Isakson of Georgia, Lisa Murkowski of Alaska and Tim Scott of South Carolina – are undecided on whether to throw their support behind Labor Secretary nominee Andrew Puzder.

A separate source-based report by CNN says the four senators have informed GOP leadership that they are withholding their support for Puzder. The four lawmakers are members of the Senate Health, Education, Labor and Pensions Committee, which Puzder is expected to appear before on Thursday for a confirmation hearing after the session was delayed multiple times.

Democrats, as they have with Mnuchin and other Trump nominations, have nearly universally expressed concern about Puzder heading the Labor Department.

In a Monday letter to Puzder, Sen. Elizabeth Warren, D-Mass., said a review of his tenure as the head of CKE Restaurants, Inc. – which oversees the Hardee's and Carl's Jr. fast-food brands – "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."

Puzder also has faced scrutiny over the revelation he formerly employed as a housekeeper a person who was not legally authorized to work in the U.S., and for allegations of abuse leveled against him by his ex-wife, which she has since retracted. According to Politico, members of the Senate HELP committee, charged with vetting Puzder, have been able to see an old episode of "The Oprah Winfrey Show" in which Puzder's former wife, Lisa Fierstein, appeared to talk about the allegations in disguise.

Many Republicans, meanwhile, have voiced support for Puzder, as have organizations such as the U.S. Chamber of Commerce and the National Council of Chain Restaurants.

"For myself, I think Andy Puzder is an outstanding choice. I'm enthusiastically in support of him," Senate Majority Leader Mitch McConnell, R-Ky., told reporters last week. "We are always looking for a nominee who's never made a mistake. Frequently, it's impossible to find nominees who have never made a mistake."

Yet the lack of unanimous support from GOP senators signals the possibility that Puzder is being passed the mantle of most-embattled Trump nominee from Betsy DeVos, who was recently confirmed as education secretary by a razor-thin margin and only after Vice President Mike Pence cast a historic tie-breaking vote.

Republicans control 52 seats in the Senate and need at least 51 votes to confirm Puzder should his nomination come to the full chamber, meaning the defections of more than two GOP lawmakers, absent the support of any Democrats, would spell doom.

Notably, two of the senators withholding their support from Puzder – Collins and Murkowski – broke party ranks to vote against DeVos.

For some lawmakers, however, a refusal to comment on backing a candidate prior to a confirmation hearing can be procedural, and Collins – along with spokespeople for Isakson, Murkowski and Scott – indicated to the Post that Puzder's hearing would play a key role in whether the lawmakers would support his nomination. Isakson and Scott aides, in particular, cited the lawmakers' practice of not deciding or commenting on nominees ahead of a hearing, though there have been exceptions at least for Scott.

A Collins aide tells U.S. News the senator also typically waits until a hearing occurs to decide on a nominee.

Puzder's confirmation process has been upheld because he delayed submitting required finance-related paperwork. At the end of January, Sen. Patty Murray, D-Wash., sent a letter to Puzder expressing concern that his paperwork had still not been submitted, forcing lawmakers to repeatedly push back the date of his hearing.