Rep. Justin AmashJustin AmashGreens take climate fight to GOP town halls US pressure on Saudis can help promote peace in Yemen Who will replace Chaffetz on Oversight? MORE (R-Mich.), a Tea Party favorite, is facing a primary challenge from a local businessman who alleges the libertarian-leaning congressman has "turned his back on our conservative principles."

Brian Ellis says he's running because Amash voted against Rep. Paul RyanPaul RyanMcConnell signals Republican-only path on tax reform Georgia campaigns keep up pressure ahead of runoff vote Meet the centrist trying to strike a deal on healthcare MORE's (R-Wis.) budget and voted "present" on bills that would have approved the Keystone XL pipeline and halted funding for Planned Parenthood.

“You have to work within your party and across the aisle. This is a democracy; it's not a dictatorship — it's not his way or the highway,” Ellis told The Hill in an interview, citing Amash’s frequent votes against his party’s leadership.

Ellis, a financial adviser in Grand Rapids, is seeking to portray himself as the more conservative candidate, though Amash’s votes against his party often come from the right.

“I'm the conservative Republican in the race,” says Ellis, who is also a member of the local school board. 

“I will vote to cut $5 trillion in spending. I will vote to cut taxes for small business, to have the Keystone pipeline approved. He can try to paint me in whatever way he wants to paint me, but the facts are going to be hard to dispute.”

Amash’s campaign downplayed any concerns about a challenge from Ellis.

“It's bizarre,” says Amash spokesman Will Adams. “Let's just say we're not worried.”

Amash, who explains all of his votes on his Facebook page, said he voted "present" on the Keystone and Planned Parenthood bills because he opposes any bills that target a specific organization, as he believes they are unconstitutional. 

He voted against Ryan’s budget because it didn’t go far enough in cutting government spending.

Amash is well-known for bucking his party. 

He was one of a handful of Republicans to vote to oust Speaker John BoehnerJohn BoehnerLobbyists bounce back under Trump Business groups silent on Trump's Ex-Im nominee Chaffetz won't run for reelection MORE (R-Ohio) at the beginning of the year, shortly after he was stripped of his committee assignments by GOP leadership. 

Amash also supported Republican efforts, unpopular among House centrists, to link funding of the federal government to defunding ObamaCare, a strategy that has led to the ongoing government shutdown. 

His push to eliminate the National Security Agency’s domestic spying program drew bipartisan support and nearly passed the House, dramatically raising his national profile.

The sophomore congressman would be hard to defeat despite his iconoclastic voting history, say a number of Michigan Republicans. 

He has strong Tea Party and libertarian support, and those groups tend to turn out in much higher numbers than establishment Republicans during GOP primaries. 

Amash has improved his standing somewhat with local voters since his first run in 2010, but he remains a polarizing figure and is disliked by some in his district’s business community.

 “I'm a little skeptical,” says Bill Ballenger, a Republican former state representative and the editor of the trade publication Inside Michigan Politics, when asked about Ellis’s chances.

“If the guy can raise a lot of money, he could make some trouble for Amash. But Amash has already overcome challenges he faced initially when he ran in the Republican primary in 2010 from moderately conservative Republicans who had a lot of backing from the business community. He's going to be very tough to beat.”

Ellis said he might be willing to put “some skin in the game” by giving his campaign some money but that most of it would be funded by donations.

Amash won in 2010 with 40 percent of the Republican primary vote after two business Republicans split most of the remainder of the vote. He did not face a primary challenger in 2012.

While some members of the business community in the district aren’t fans of Amash’s libertarianism, others are supporters who will help pull in local support. 

They include the DeVos family, the wealthy owners of Amway, who are major players in western Michigan GOP politics. 

“Justin Amash has continued to strengthen his base of support among primary voters in the district and isn't beatable in 2014,” says John Yob, a top Michigan GOP strategist who ran one of Amash’s opponent’s campaigns in 2010 but has since developed a relationship with the congressman.

“Ellis may earn some support from a few key donors, but that isn't representative of the broader electorate and ultimately endorsements don't win elections.”

Michigan state Sen. Mark Jansen (R) is also weighing a bid for the seat.

Amash has not traditionally been a stellar fundraiser, and had just $164,000 cash on hand as of the end of June. 

But he might be able to generate some campaign cash quickly given his national stature and the support of a wide-ranging network of libertarian activists associated with former Rep. Ron Paul (R-Texas) and Sen. Rand PaulRand PaulWe can put America first by preventing public health disasters Conservative activists want action from Trump McConnell: 'Big challenge' to pass ObamaCare repeal in Senate MORE (R-Ky.).

In a fundraising plea on Facebook in September, Amash asked for contributions because the “Washington political class is scheming to take me out.”

—This story was originally posted at 9:17 a.m. and has been updated.