Campaign ads featuring six Gosar siblings condemning their brother and endorsing his opponent, David Brill, first went online on Friday.
"It would be hard to see my brother as anything but a racist", Mr Gosar's sister Grace tells voters in another upcoming advert, according to The Phoenix New Times.
David Gosar tell viewers, "Paul's absolutely not working for his district".
Six siblings of Arizona Republican representative Paul Gosar appeared in a campaign ad endorsing his Democratic opponent, David Brill.
The siblings ad, released Friday, already has nearly 400,000 views.
His House seat is not hotly contested; he won by 40 points in the last election.
A campaign staffer for Brill spied his tweets, which led to a conversation between David Gosar and the campaign, and then a trip to Wyoming to film the ads with the Gosar siblings earlier this month.
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"To speak up against my brother, it brings sadness to me", Joan Gosar said. Gosar has also spread a conspiracy theory that the "Unite the Right" rally in Charlottesville, where one woman was killed by a self-identified white supremacist, was actually plotted by left-wing organizers to damage Trump.
In January, he drew bipartisan rebukes after he said that he asked the Capitol Police and Attorney General Jeff Sessions to check IDs at the State of the Union to arrest and deport any undocumented immigrants in attendance.
"I greatly appreciate and admire the courage of the Gosar family in stepping into the national spotlight for this vital message", he said. Paul Gosar is the oldest of ten siblings. Another said he traded in everything he learned around the family kitchen table.
Gosar now represents a sprawling district in northeastern and central Arizona.
Jennifer Gosar talks about her brother Paul in an advertisement for his opponent David Brill. He told the Phoenix New Times he is grateful for the Gosar family's support, calling them "upstanding, wonderful people".
Bernadette said that she has a "wonderful" family but that the members of the family don't always see eye to eye politically. The Republican is considered likely to keep the seat he has held since 2011.
Gosar is also a strong proponent of Trump's anti-immigration and anti-refugee policies.