Washington (CNN) Four Republican senators have told GOP leadership they are withholding support for 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.

Fast food executive Andrew Puzder 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.

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