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.

READ: Ryan: House Republicans should vote ‘conscience’ on Trump

Johnson’s shift comes as a chorus of Republicans have either distanced themselves from Trump or outwardly said they will oppose him in November. Some incumbent Republicans facing tough re-elections, like Illinois’ Mark Kirk, have said they cannot even support Trump. Richard Armitage, a former top aide to President George W. Bush, announced last week he would vote for Hillary Clinton.

House Speaker Paul Ryan – who has maintained his endorsement of Trump but become one of his most high-profile critics in the party – offered cover to other Republicans, saying that he would understand anyone who could not support Trump because of their “conscience.”

And while Trump is the presumptive GOP nominee, there are currently efforts underway by some Republicans to upend his nomination, something Johnson may have hinted at Sunday.

“It’s been my intention to support the nominee. And again, nobody can predict the outcome of this thing. So I think things remain reasonably uncertain,” he said.