Heck, who is running for the Senate seat being vacated by Senate Minority Leader Harry Reid (D-NV), earlier this month called on Trump to drop out of the race following a 2005 tape in which Trump makes vulgar comments about women. He said at the fundraiser that Trump has put him in a tough spot.
"I want to support him, I really do," Heck said, according to CNN. "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 percent or 30 percent of the Republican base."
Heck said that Trump had one more chance to turn the race around at the debate this week.
"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," he told the audience at the fundraiser.
Heck said that the 2005 tape alone did not prompt him to change his mind on Trump, lamenting that the Republican presidential nominee can't stay on message.
"Look, it wasn't this video that made the final decision, alright? He's had this pattern of behavior that's persisted... even while he was on the campaign," Heck said. "It's one thing if it happened 11 years ago and now he's concentrating on the issues and talking the way we need to talk to him about it."
He said that Trump was not reaching out to a broad enough voter base with his combative campaign style.
"Alright, if he's attacked he wants to punch back. He wants to punch back hard. Look what he's doing is shoring up the Republican base and that's great, but you know cannot win a federal race nationwide with just the Republican base, you've got to have a big tent and he doesn't. And that's why he's floundering in the polls," Heck said at the fundraiser, according to CNN.