A candidate for U.S. Senate in Massachusetts who describes himself as a “real Indian” is suing the city of Cambridge for demanding he remove his signs that say his challenger, Sen. Elizabeth Warren (D-MA), is a “fake Indian.”

Shiva Ayyadurai, who is running for the Senate as an independent, filed a federal lawsuit Sunday accusing the city of Cambridge of violating his First Amendment rights under the Constitution for demanding he remove his signs attacking Warren for her unfounded claims of Native American ancestry, the Washington Times reported.

Ayyadurai’s campaign bus, which has been parked for more than a month in front of an office building he owns, displays signs depicting the progressive Massachusetts senator in an Indian headdress next to a picture of himself with his campaign slogan: “Only a REAL INDIAN Can Defeat the Fake Indian.”

But this month, Cambridge building inspector Branden Vigneault sent Ayyadurai’s campaign a notice that said he lacked proper permitting, threatening a $300 per day fine and legal penalties if the campaign does not remove the signs.

Ayyadurai’s campaign argues the building code does not apply to his images because they are posted on a bus and not a brick-and-mortar structure.

“We will not remove the slogan from our bus,” Ayyadurai told the Times. “We will defend the First Amendment, and we will fight this egregious attack on the First Amendment, at any cost.”

Ayyadurai has made it a hallmark of his campaign to challenge Warren’s claims of Native American ancestry, which no documentation has proven.

As proof of her heritage, Warren pointed to a “family newsletter” from 2006, uncovered by an amateur genealogist. It stated that her third great-grandmother was a Cherokee Indian.

Warren’s campaign also claimed that the Massachusetts senator is Native American because her cousin Candy Rowsey edited a cookbook called Pow Wow Chow. A Boston talk radio show host found in 2012 that Warren may have plagiarized two recipes that she submitted to the cookbook.

Ayyadurai, meanwhile, is using the lack of documented evidence of Warren’s heritage to his advantage.

He pulled a campaign stunt in July 2017 where he sent Warren a 23&Me DNA ancestry test through Amazon, which she refused. Ayyadurai then posted a receipt on Twitter of his refunded purchase and wrote that he was “deeply saddened” the progressive Massachusetts senator refused his gift.