The shifts, Heller's Republican and Democratic opponents say, suggest he is operating out of fear -- first worried about not looking like a moderate, and then looking too much like a moderate.
Tarkanian pointed to the June news conference with Sandoval, where Heller said he'd vote against any bill that slashed Medicaid funding or reduced access.
"Now, all of a sudden, he's in favor of a bill that does exactly that and I think cuts it even more?" Tarkanian said. "What you see here is that Dean Heller doesn't have any convictions for the positions he holds and he's willing to change them depending on who he's speaking in front of."
"He's stuck between Donald Trump and Danny Tarkanian, and he feels boxed in," Rosen said. "So he's going to try to please whatever he thinks is going to be his particular base and his particular donors, instead of doing what's right."
"I think it's thoughtless. I think it's heartless. I think that when people come and talk to him, he really isn't listening," Rosen added.
Asked by CNN to respond to the criticism he's facing, Heller's office pointed only to the bill's increased state-level flexibility when compared to earlier GOP measures as how Nevada would make up for reduced long-term Medicaid spending.
In a statement, Heller called the bill "the best solution to repeal and replace Obamacare," and touted the flexibility of its block grant system, saying it would allow individual governors to devise their own strategies to cover Medicaid recipients. His aides also said his support for the bill is unrelated to the 2018 campaign.
"Sen. Heller has a long record of voting to repeal and replace Obamacare, and he supported the Graham-Cassidy plan in July because it was -- and continues to be -- the best solution on the table for the state of Nevada," Heller spokesman Keith Schipper said in an email Thursday, after CNN had published Tarkanian and Rosen's comments. "On the other hand, perennial candidate Danny Tarkanian now apparently opposes the repeal of Obamacare and Jacky Rosen won't tell Nevada voters if she supports the only alternative to Obamacare that her party leaders are championing: Sen. Sanders' socialized healthcare plan."
Heller's health care reversals
For years, Heller stuck to the Republican position that Obamacare should be repealed and replaced.
But when Republican congressional leaders attempted to do so this summer, Heller sided with Sandoval. Heller told reporters that to get his support, "you've got to make sure the Republican governors that have expanded Medicaid sign off on it. I've been saying that for months."
"Where is Gov. Sandoval? What does he think? How does he feel about the changes that are occurring?" Heller said then.
Sandoval didn't back the GOP bill at the time -- the so-called "skinny bill" -- but Heller voted for that measure anyway
while offering up a non-binding resolution to express support for Obamacare's Medicaid expansion. (The "skinny bill" failed in a dramatic late-night vote.)
His shift was punctuated by a White House meeting in which Trump said he thought Heller would bow to political pressure and vote to repeal Obamacare.
"He wants to remain a senator, doesn't he?" Trump said.
Now, he is a co-author, along with South Carolina Sen. Lindsey Graham, Louisiana Sen. Bill Cassidy and Wisconsin Sen. Ron Johnson, of a bill that would drastically overhaul Medicaid, turning the program into a block grant in which states would get fixed amounts of federal funding each year.
But Sandoval isn't with him, despite the White House's efforts -- including a one-on-one meeting with Vice President Mike Pence and top administration health officials at this summer's National Governors Association meeting -- to win him over.
In a bipartisan letter with other nine governors, and then in his own statement, Sandoval announced his opposition to the Graham-Cassidy bill Tuesday -- leaving Heller without his most important home-state political ally.
Heller's opponents say his twists and turns on health care show he was acting out of fear -- first of the general election, and now of a primary.
Rosen, the Democratic front-runner in the race for Heller's seat, pointed to Sandoval's position and said Heller is pushing a bill that would "make people sicker" and "cause medical bankruptcy."
"Gov. Sandoval is pleading with his friend Dean Heller to pay attention to what he's been trying to do to protect our families, and every family," Rosen said.
Why Heller supports Graham-Cassidy
What's changed since June, when Heller so closely linked himself to Sandoval?
Heller points to "flexibility," arguing it would allow states a much freer rein to design their own ways to cover the Medicaid population.
"Our plan moves decision-making to the states and provides each state the flexibility needed to innovate, develop, and implement new options to bring down costs and increase coverage. Our plan also allows states to maintain programs that are currently working for them, protects individuals with pre-existing conditions, and eliminates the individual mandate penalty because people who can't afford a product their government forces them to buy should not have to pay a fine," Heller said.
"Most importantly, Nevada wins under Graham-Cassidy-Heller-Johnson because it would be receiving more federal funding under this proposal and increased flexibility to help make sure people have access to quality care," he added.
However, the bill allows states to drop protections for those with pre-existing conditions, and analyses have found it actually slashes Medicaid funding, particularly for states like Nevada that expanded the program under Obamacare.
Asked to clarify Heller's claim of "more federal funding," his office said that comparison is to current funding under Obamacare.
However, that doesn't take into account medical inflation, population growth or other factors typically used to project programs' long-term costs.
An analysis from the left-leaning Center on Budget and Policy Priorities
found Nevada would face a steep cut in federal funding, receiving $639 million less in the year 2026 under the Graham-Cassidy bill than it would under current law.
The nonpartisan Congressional Budget Office -- which provides the analysis on which Democrats and Republicans typically rely in examining bills' consequences -- has not released a detailed analysis of the bill and will not have one prepared by next week, when Senate Majority Leader Mitch McConnell, R-Kentucky, says he intends to hold a vote.
Heller's office also noted that Nevada would not be on the hook for the 10% match states that expanded Medicaid under Obamacare are require to supply starting in 2020, and could use that money to help cover its Medicaid recipients.
This story has been updated.