HARRISBURG, Pa. (AP) — Mehmet Oz’s rivals in Pennsylvania’s competitive Republican primary for U.S. Senate are escalating their attacks on the celebrity heart surgeon’s connections to his parents’ native country of Turkey, raising it as a possible national security issue.

Oz, best known as TV’s Dr. Oz, has rejected any suggestions he is a threat to national security and accused an opponent, ex-hedge fund CEO David McCormick, of making “bigoted attacks.” If elected, Oz would be the nation’s first Muslim senator, although Oz has not campaigned on that milestone.

The criticism of Oz and his ties to Turkey has mushroomed in the weeks after Oz won the endorsement of former President Donald Trump, who remains popular with conservative voters. With the state’s May 17 primary in sight, Trump held a rally with Oz in western Pennsylvania on Friday evening, coming off a big win in Ohio’s Republican Senate primary for Trump’s candidate, JD Vance.

Oz, who was born in the United States, holds Turkish citizenship, served in Turkey’s military and voted in its 2018 election, but says he would renounce his dual citizenship in Turkey if he is elected.

Trump’s former secretary of state and CIA director, Mike Pompeo, who endorsed McCormick in the race, told reporters Friday that Oz owes an explanation about the “scope and the depth of his relationship with the Turkish government.”

Americans should know if Oz is “fit for duty,” Pompeo said.

As CIA director, Pompeo served side-by-side in the Trump administration with Michael Flynn, Trump’s first national security adviser who drew Justice Department scrutiny because of lucrative consulting work he and his firm did that benefited the Turkish government.

Oz has financial ties to Turkey, as well.

In his financial disclosure report to the Senate, Oz disclosed property that he owns in Turkey, assets from his late father’s estate that are tied up in legal proceedings there and an endorsement agreement with Turkish Airlines, which is partly owned by the Turkish government.

In recent debates, McCormick — a decorated U.S. Army combat veteran of the Gulf War — has accused Oz of unnecessarily holding dual citizenship in Turkey and tried to contrast Oz’s service in the Turkish military with McCormick’s in the U.S. Army.

Another rival, Carla Sands, Trump’s former ambassador to Denmark who inherited a commercial real estate fortune, has suggested Oz has dual loyalties, calling him “Turkey First,” as a play on Trump’s “America First” governing philosophy.

Fending off McCormick’s attacks in March, Oz suggested that his religion is being targeted, accusing McCormick of making “bigoted attacks” that are “reminiscent of slurs made in the past about Catholics and Jews.”

Oz has maintained that he served in Turkey’s military as a young man to keep his dual citizenship. He keeps it to this day, he said, so he has legal power in Turkey to make health care decisions for his mother, who has Alzheimer’s disease.

Oz voted in Turkey’s 2018 election when he was at the consulate in New York for meetings about his humanitarian work on behalf of Syrian refugees in Turkey, his campaign said.

He voted against Turkish President Recep Tayyip Erdogan, his campaign said, noting that it is not unusual for Americans with dual citizenship to vote in elections in other countries.

“Voting in an election is far different from being actively engaged in the political work of the Turkish government, which Dr. Oz has never been involved with,” Oz’s campaign said.

Senate historians have been unable to find a U.S. senator who maintained dual citizenship.

Trump, in an hourlong speech at Friday night’s rally, attacked McCormick for the first time by name, saying he’d been with a company that “managed money for communist China.”

McCormick is, Trump said, the “candidate of special interests and globalists and the Washington establishment” spending millions of dollars to defeat Oz and “ripping off the United States with bad trade deals and open borders.”

Trump did not specifically mention attacks on Oz’s ties to Turkey, but boosted Oz to the crowd as a “warrior” and a longtime friend who has the best chance of winning the battleground state seat in the fall general election.

David Laufman, the former chief of the counterintelligence section at the Department of Justice’s national security division, said he thinks of a national security concern as individuals and organizations that present terrorist threats, cybersecurity threats or economic security threats, or are involved in influence operations directed at the U.S. on behalf of foreign powers.

“I think we need to be careful about categorizing any American as a national security risk simply because of their ties to a foreign country,” Laufman said in an interview.

Edward Ahmed Mitchell, deputy director of the Council on American-Islamic Relations, said the nonprofit does not comment on specific campaigns.

But in general, he said, the organization has seen attacks on one aspect of a candidate — such as their place of birth — as a stand-in for a more obviously racist attack, such as on their race or religion.

Flynn — Trump’s former national security adviser — was ousted in the first month of the Trump administration after the White House said he lied about his Russian contacts during the transition period.

He later admitted in a criminal case brought by special counsel Robert Mueller that his filings to the Justice Department when he registered as a foreign agent for his Turkish work included “false statements and omissions.”

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Associated Press writer Eric Tucker in Washington contributed to this report.

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