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Four Republican senators have told GOP leadership they are withholding support for Andrew Puzder, President Donald Trump’s choice for labor secretary, setting off an intense effort by Majority Leader Mitch McConnell and business groups to bring at least two back into the fold so that the nomination does not fail, several sources involved in the effort tell CNN.

The four, these sources say, are GOP Sens. Susan Collins of Maine, Lisa Murkowski of Alaska, Tim Scott of South Carolina and Johnny Isakson of Georgia.

Puzder, a fast food executive, is the President’s choice, and among the last Cabinet picks to get a confirmation hearing because of controversies that include an admission he employed an undocumented housekeeper. His hearing, delayed several times, is now scheduled for Thursday.

Republicans are not counting on any Democratic votes for Puzder. So, with a 52-48 majority, they would need to hold at least 50 Republicans, as was the case last week when Vice President Mike Pence was called on to break the tie and advance the confirmation of Betsy DeVos as education secretary.

Murkowski and Collins were the two GOP defections on the DeVos vote, and of the four who have voiced reservations about Puzder are viewed as the most difficult to get back in the fold.

If at least two of the four reluctant Republicans cannot be swayed, McConnell could face the unwelcome task of advising the White House to pull the nomination instead of facing an embarrassing Senate floor defeat. But the sources involved in the urgent lobbying effort, speaking to CNN on condition of anonymity, said the leader viewed this as a test of party unity and his leadership, and was determined to round up the necessary votes.

To that, several business organizations with political clout have joined the effort, including the Chamber of Commerce, which sent lawmakers a letter supporting the Puzder nomination last week and has been working in concert with Senate GOP leadership to sway the senators who have reported their opposition or likely opposition to Puzder.

Collins told CNN she still has not made a decision about whether to support Puzder and is reviewing material about the nominee including an appearance his ex-wife made on Oprah. She insists she won’t make up her mind until after Thursday’s hearing.

“I have gone to review the Oprah show for an hour on which his former wife appeared and I’m reviewing the other information that has come to light. I’m sure all of this will explored fully,” she said.