The GOP tax bill picked up support from two more senators Monday and resumed its lurch toward passage this week, despite last-minute concerns when another key senator raised questions that seemed to put his support in doubt.

Sen. Susan Collins of Maine and Sen. Mike Lee of Utah, two holdouts whose votes were likely but uncertain, offered strong endorsement of the package.

But Sen. Bob Corker, R-Tenn., who was the sole GOP vote against the original Senate bill and then endorsed the final package only last week, expressed concerns over the weekend about a provision that would provide tax breaks for real estate developers.

Critics were quick to note that many lawmakers with sizable real estate holdings, including Corker, as well as President Donald Trump, would personally benefit from the provision. Corker spent the weekend fending off attacks that he had traded his vote.

By Monday, Corker's office confirmed he would still support the tax bill, but he asked GOP leaders in the Senate for an explanation over how the provision made it into the bill.

"Because this issue has raised concerns, I would ask that you provide an explanation of the evolution of this provision and how it made it into the final conference report," Corker wrote Sunday in a letter to Sen. Orrin Hatch, R-Utah, chairman of the Finance ­Committee.

The vote will be close. Sen. John McCain, who is battling brain cancer, has returned home to Arizona.

Republicans can afford to lose only two votes in the Senate under special budget rules, given their 52-48 majority.

Tribune Washington Bureau