Tea party favorite and Nebraska Senate candidate Ben Sasse is having trouble insulating himself from the rage of the tea party.

It’s an odd situation for someone like Sasse, who’s been endorsed by the Senate Conservatives Fund, the Club for Growth, and FreedomWorks — which originally backed Sasse’s opponent, former state Treasurer Shane Osborn (R) — and Sen. Mike Lee (R-UT). At the moment, he’s considered one of the more likely candidates favored by the hard right to win a Senate seat this cycle. Sasse can also proudly tout to conservatives that he got a stern talking-to from Senate Minority Leader Mitch McConnell (R-KY) for associating with the Senate Conservatives Fund — which has made McConnell its number-one target.

But increasingly, Sasse is being forced to watch his right flank. On Thursday, Sasse’s campaign sent out a new fundraising letter authored by campaign manager Tyler Grassmeyer hitting the first negative ad released by Osborn’s campaign which attacked Sasse for working as a government bureaucrat. Sasse served as U.S. Assistant Secretary of Health and Human Services under President George W. Bush.

“The ad is filled with dishonest attacks, but the most blatantly false is that “[Sasse served] nearly a decade as a government bureaucrat,” Grassmeyer wrote. “Ben Sasse served President George W. Bush twice, for a total of three and a half years and Rep. Fortenberry for six months. Four years is less than ten years, and when the President asks you to serve, you do it.”

Part of the 2014 election cycle has involved candidates elected on past tea party waves earning the ire of the tea party, such as Sen. Marco Rubio (R-FL) or Rep. Renee Ellmers (R-NC), who faces a primary from Internet talk show host Franke Roche. Walking this fine line between tea party and establishment has become all too common for candidates theses days, and shows that how “tea party” someone is really comes down to a matter of opinion.

Sasse’s fielded a few attacks that would seem more fitting for an establishment candidate than a tea partier. About a week earlier, a group of 52 Nebraska conservative activists wrote a letter saying Sasse was not one of them.

“Our philosophies and interests slightly differ in some ways but on this we can all agree: Ben Sasse for US Senate is NOT the choice of conservative, libertarian, and tea party movement activists and group leaders in Nebraska,” the letter read. “We were not consulted, polled, or contacted by these Washington DC groups such as FreedomWorks has claimed.”

And when FreedomWorks did switch its endorsement, the group’s vice president for public policy, Dean Clancy, resigned in protest. Clancy did not explain his decision to leave the organization but he previously wrote a blog post defending FreedomWorks’ decision not to endorse Sasse (before the group switched).

Sasse, unlike a number of tea party candidates, has also won support of more establishment-type Republicans, like House Budget Committee Chairman Paul Ryan (R-WI) and retiring Sen. Tom Coburn (R-OK). A big knock on Sasse among conservatives has also been past comments on health care reform. He seems to have flip-flopped his position on Medicare Part D. In an interview with Slate’s Dave Weigel Sasse said he had always opposed the Medicare prescription drug benefit. The thing, though, is that Sasse argued in an op-ed in U.S. News and World Report that the nation should use Medicare Part D as a type of model for reforming and improving the country’s entire health care system.

It’s hard to say what exactly is holding Sasse back from completely dominating the race. Some polling has the primary in a virtual dead heat with Osborn and Sasse has also managed to out-fundraise the former state treasurer. But it’s unclear if he’ll be able to pull ahead, despite all the support and money. Sasse, despite modeling himself as a tea party candidate, has actually benefited from his establishment backgrounds and connections, Aaron Trost, the former campaign manager for Sen. Deb Fischer (R-NE), told TPM.

“A lot of typical anti-establishment candidates, they don’t have a lot of political connections but I think because of Ben’s background of maybe working in the Bush White House, having a few contacts in D.C., I think he’s had a lot of political relationships and obviously those relationships have been helpful in gaining support for his Senate campaign,” Trost told TPM.