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U.S. Sen. Lindsey Graham speaking with reporters in February. Graham sent a letter this week to FBI Director Christopher Wray to ask about the agency's different handling of alleged foreign agents working for U.S. Sen. Dianne Feinstein, D-Calif., and President Donald Trump's campaign. File/Wade Spees/Staff

U.S. Sen. Lindsey Graham continued to raise questions Friday about the FBI's approach to President Donald Trump's 2016 campaign, asking the agency's director why they informed a Democratic California senator about Chinese ties to one of her staffers but did not take similar steps for the Republican presidential candidate.

Amid reports that U.S. Sen. Dianne Feinstein, D-Calif., was warned years ago that one of her aides was a suspected Chinese spy, Graham wrote FBI Director Christopher Wray asking for more details about how the issue was handled.

While Graham, R-S.C., said he appreciated the FBI's efforts to identify the Feinstein staffer with potential ties to the Chinese government and warn her about it, he said he was "deeply troubled" the agency did not take similar action when suspicions were raised in the Trump campaign sphere.

"It appears that rather than provide then-candidate Donald Trump or Trump Campaign officials with a defensive briefing to inform them of the FBI's concerns, the Bureau opted to use a confidential informant and a dossier funded by the Democratic National Committee to launch an unprecedented counterintelligence investigation into the Trump Campaign," Graham wrote.

He cited a Fox News report from Aug. 7 that said the differing treatment between Feinstein and Trump had spurred claims of a "double standard" — namely from House Intelligence Committee Chairman Devin Nunes, R-Calif., and the Wall Street Journal editorial board.

A retired FBI special agent told Fox News the discrepancy could be due to the difference in scope of the two investigations.

While the concern with Feinstein was limited to a single staffer, ex-FBI spokesman John Iannarelli said, "The Trump investigation appears to be much broader."

Graham asked Wray three main questions:

  • Why did the bureau brief Feinstein about a threat from one of her staffers but not Trump?
  • What steps did the FBI take to inform Trump about possible improper ties between his staffers and the Russian government?
  • What policies does the FBI have in place to warn current and future political candidates about foreign efforts to infiltrate their campaigns?

The FBI did not immediately respond to a request for comment on the letter.

Once a vocal critic of Trump, Graham has become a loyal ally and attack dog for the president in recent months. 

He has called for a special counsel to investigate the Department of Justice and the FBI for connections to Fusion GPS, the firm that hired former British spy Christopher Steele to compile the controversial “Steele dossier.”

He declined to sign onto a resolution condemning Trump's tariffs, saying he wants to give the president room to negotiate.

He defended Trump's decision on Wednesday to revoke the security clearance of former CIA director John Brennan, a prominent critic of the president.

And he applauded the FBI for firing agent Peter Strzok this week over text messages that were critical of Trump.

Now a regular golf partner of Trump's, Graham tells critics that his cooperative relationship with the president benefits the country and his home state of South Carolina.

"If you don't like me working with President Trump to make the world a better place, I don't give a (expletive)," Graham said in June on CNN.

Trump highlighted Graham's pressure on the FBI in a tweet last week after Graham said on Fox News that there was a "double standard."

Follow Jamie Lovegrove on Twitter @jslovegrove.

Jamie Lovegrove is a political reporter covering the South Carolina statehouse and congressional delegation. He previously covered Texas politics in Washington for The Dallas Morning News and in Austin for the Texas Tribune.