Sen. Dean Heller of Nevada became the fifth Senate Republican on Friday to announce opposition to the Senate Republican health care plan to repeal and replace Obamacare, putting the legislation's passage in jeopardy ahead of a vote next week.

"This bill, that's currently in front of the United States Senate is not the answer. It's simply not the answer," Heller said at a press conference in Las Vegas with Nevada Gov. Brian Sandoval, a Republican. "I'm announcing today that in this form, I will not support it."

Heller said he can't support a piece of legislation that likely would take away health care coverage from millions of Americans and tens of thousands of Nevadans.

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As a result of his opposition, pro-Trump organization America First Policies is launching a TV ad against Heller, CBS News has learned. Heller is up for re-election in 2018. He is considered the most vulnerable Republican senator running next year, and a high-profile Democrat is reportedly set to challenge him: Nevada Rep. Jacky Rosen.

"It's going to be very difficult to get me to a yes," Heller said Friday. He added, "You have to protect Medicaid expansion states. That's what I want," however, he acknowledged that changing the bill in order to do that is "going to be a very difficult lift" because of the conservative senators who are against the Medicaid expansion.

Sandoval agreed with Heller, saying, "The current bill as written is something that needs to change." Sandoval was among a number of Republican governors who opted into a Medicaid expansion under Obamacare. Before he made that decision, Sandoval said Friday that his state's uninsured rate stood at 24 percent. Now, he says that only 11 or 12 percent of Nevadans are uninsured. Before the expansion, he said 18 percent of children were without health insurance.

"We're now at 8 percent," Sandoval said Friday.

Hours after text was revealed Thursday, Sen. Rand Paul, R-Kentucky, announced with three other Senate Republicans that they opposed the bill in its current form because it doesn't go far enough in repealing President Obama's health care law. Three "no" votes would put the bill's passage in jeopardy.

Republicans actually need 51 votes to pass the legislation, but one of them can be Vice President Mike Pence's tie-breaking vote. Taking that into consideration, and assuming all Democrats vote against it, Republicans can only afford two defections. The Senate currently has 52 Republicans and 48 Democrats.

CBS News political director Steve Chaggaris contributed to this report.