WASHINGTON — A day before Republicans are expected to begin voting on their $1.5 trillion tax cut, the big question was not whether it would pass but why the lone Republican Senate holdout, Bob Corker of Tennessee, suddenly flipped his position to support a bill he once said was fiscally irresponsible.

The theories varied from political to financial. Some suggested Mr. Corker, who has said that he will not seek re-election to the Senate, may be rethinking his political future, while others claimed he was bought off by a late-added provision that would benefit people with large real estate holdings, including him.

In an interview on Monday, Mr. Corker dismissed those theories and said he faced a wrenching decision as a Republican lawmaker with deep concerns about the country’s mounting debt and a strong desire to overhaul the tax code. In the end, he said, he put his fiscal principles aside on the assumption that the nation would be better off with the tax cuts than without.

“It’s been really tough, especially because I did think, I really felt like we could have had a bipartisan bill that would have really withstood more fully the test of time,” Mr. Corker said.