The state lawmaker and retired Army colonel is such a polarizing figure that even prominent members of his own party have thrown their support behind Democratic rival Josh Shapiro, citing Mastriano's extremist views.
With just over two months until the Nov. 8 general election, Mastriano stands out as an example of why the upcoming election might not produce as many victories as Republicans had hoped for going into the 2022 cycle.
Republican voters across the United States have nominated a number of candidates who lack political experience and often hold far-right views that may not appeal to moderate voters in some of the most consequential races in November.
But the stakes are particularly high in Pennsylvania, a political battleground state that helps decide congressional and presidential elections.
The winner of the open governor's race will choose the state's top elections official who will oversee its 2024 presidential election, and will also have the power to block or advance efforts by the Republican-led state legislature to severely restrict abortions.
Unlike candidates in some other competitive races, Mastriano has shown little interest in tempering his views to court Pennsylvania's crucial moderate voters.
He has called legal abortion “a national catastrophe” and has promised to push a ban on abortions after six weeks of pregnancy if elected governor.
A supporter of former President Donald Trump's false claims of a stolen 2020 election who was outside the U.S. Capitol during the Jan. 6, 2021, attack, he also has pledged to take the extraordinary step of requiring people to “re-register” to vote — a move that violates federal law, scholars say - and to decertify certain voting machines.
Trump is scheduled to appear at a rally in Pennsylvania on Saturday with Mastriano and other candidates he has endorsed.
But a growing number of Republicans, including former Homeland Security Secretary Michael Chertoff this week, have taken the unusual step of publicly denouncing Mastriano's candidacy and backing Shapiro, the state's attorney general.
Jim Schultz, a Philadelphia lawyer who served in Trump’s White House, published an opinion piece on Monday in support of Shapiro. Two days later, Schultz held a fundraiser for the Democrat at a Philadelphia restaurant that included several prominent Republican donors, according to a source familiar with the event.
“I am a conservative, and I will continue to support principled conservative candidates and elected officials," Schultz said in an interview, declining to comment on the fundraiser.
"Doug Mastriano is not a principled conservative. He is an extremist who has supported conspiracy theories and will only serve to divide our party and commonwealth."
Mastriano, who until recently barred all news media from his campaign events and typically grants interviews only to outlets that share his far-right politics, did not respond to requests for comment.
One of his supporters, voter Dan Sardella, defended the candidate even though he said he wished Mastriano would stay away from the election denials and abortion.
“He may be extreme," Sardella said, "but maybe that’s what’s needed right now."
Mastriano’s latest campaign finance report showed less than $400,000 in cash on hand as of June. The deep-pocketed Republican Governors Association so far has withheld financial support.
By contrast, Shapiro is flush with cash, which he has used to fund television ads casting Mastriano as too extreme for Pennsylvania. The Democrat's campaign said it has placed a $16.9 million initial reservation on TV ads that starts next week, bringing its total spending on TV so far to $35 million.
Some August opinion polls showed Shapiro with a double-digit lead, though a survey conducted by Emerson College on Aug. 22-23 put the Democrat ahead by just three percentage points.
Six Republican county chairs in the state, who spoke on condition they not be named, expressed concern about the state of Mastriano’s campaign.
Some thought he had yet to adequately respond to a Reuters report last week that Mastriano had worn a Confederate uniform in a 2013-2014 faculty photo at the Army War College, where he was a teacher at the time.
Displays of Confederate symbols can be seen as insensitive to those who view them as painful reminders of racial oppression and the Civil War that saw Confederate states fight to keep Black people enslaved.
Mastriano provided his first public comment on the photo on Tuesday in an interview with conservative One America News, saying, "I think it’s important to understand the past to have a better future and also to not repeat the mistakes of the past."
During a recent lunchtime speech in the Philadelphia suburb of Aston, Mastriano attacked Shapiro for his crime-fighting record and his support of COVID-19 lockdowns and transgender athletes.
"And they call us extreme," Mastriano told supporters wearing MAGA hats and anti-vaccine buttons. "These people are just crazy."
Dave White, a supporter and former rival in the Republican primary, said Pennsylvania voters were just starting to pay attention to the governor's race. He noted that despite Mastriano's money disadvantage, some polls showed a close contest.
“I am confident Mastriano has the right message and resources to win this race,” White said.