Story highlights Dean Heller becomes the fifth Republican senator to say he won't back a health care bill

GOP Senate leader can only lose two members of their 52-senator caucus

(CNN) Dean Heller on Friday became the latest Senate Republican to say he opposes the current GOP health care bill.

"It's simply not the answer," the Nevada Republican said at a news conference alongside Gov. Brian Sandoval in Las Vegas. "And I'm announcing today that in this form, I simply will not support it."

Almost immediately, the pro-Trump group America First Policies decided to launch what a source with the group says will be a major television, radio and digital ad buy against Heller -- a remarkable attack on a member of Trump's own party whose seat is endangered in 2018.

It was not immediately clear whether the group intends to back a primary challenger to Heller.

Heller, like four other GOP senators who announced their opposition to the Obamacare repeal bill that was unveiled Thursday, left the door open to negotiations. But he said "it's going to be very difficult to get me to a 'yes'" on the bill.

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