Story highlights Joe Heck, who's running for Senate, gave a candid assessment of his party's presidential nominee Donald Trump

His comments came in a behind-closed-doors fundraiser last week

(CNN) Rep. Joe Heck gave his most extensive assessment of his decision to dump Donald Trump at a closed-door fundraiser last week, saying Republicans had "cut a lot of slack" to his party's presidential nominee for months but that he had become too toxic now amid an undisciplined and unfocused campaign.

According to audio of the private event obtained by CNN, Heck seemed to recognize that his decision to abandon Trump had become a sore subject among some of his core supporters, calling it a "very difficult situation." About six minutes into the recording he defended his decision to rescind his Trump endorsement by saying that the GOP nominee's "pattern of behavior" was bound to "hand" the White House to Hillary Clinton.

"I want to support him, I really do," Heck, a Nevada Republican, said during a 90-minute discussion during a Las Vegas fundraiser for his Senate campaign.

"But he has got to change his tone and he's got to be -- I don't want to make him into a politician or make him into the same thing he is running against -- but he has got to realize he is not going to win this race by appealing to the 20% or 30% of the Republican base," Heck added.

Heck, who noted that Trump had "one more debate chance" to turn around his candidacy in Las Vegas this week, added, "At the very least, if I have to go with my own conscience to make this right, I am going to vote for Michael Pence as vice president."

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