Washington (CNN) Sen. Ron Johnson, a Wisconsin Republican facing a tough re-election bid, tamped down previous comments that he would endorse the Republican nominee, saying Sunday that he was only prepared to "support" Donald Trump.

"To me, 'endorsement' is a big embrace. It basically shows that I pretty well agree with an individual on almost everything. That's not necessarily going to be the case with our nominee," Johnson told CNN's Dana Bash on "State of the Union." "I'll certainly be an independent voice. Where I disagree with a particular nominee, I'll voice it. Whether it's Hillary Clinton, Donald Trump or somebody else, I'll voice those disagreements."

Last month, shortly after Sen. Ted Cruz dropped out of the race, Johnson told The Eau Claire Leader-Telegram that he was "going to certainly endorse" Trump.

"I am going to certainly endorse the Republican nominee, and obviously it looks like that will be Mr. Trump," Johnson had said.

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