Sen. Charles Grassley, chairman of the Senate Judiciary Committee, has sent a letter to FBI Director James Comey demanding the story behind the FBI's reported plan to pay the author of a lurid and unsubstantiated dossier on candidate Donald Trump. In particular, Grassley appears to be zeroing in on the FBI's deputy director, Andrew McCabe, indicating Senate investigators want to learn more about McCabe's role in a key aspect of the Trump-Russia affair.

Grassley began his investigation after the Washington Post reported on February 28 that the FBI, "a few weeks before the election," agreed to pay former British spy Christopher Steele to investigate Trump. Prior to that, supporters of the Hillary Clinton presidential campaign had paid Steele to gather intelligence on Clinton's Republican rival. In the end, the FBI did not pay Steele, the Post reported, after the dossier "became the subject of news stories, congressional inquiries and presidential denials." It is not clear whether Steele worked under agreement with the FBI for any period of time before the payment deal fell through.

"The idea that the FBI and associates of the Clinton campaign would pay Mr. Steele to investigate the Republican nominee for president in the run-up to the election raises further questions about the FBI's independence from politics, as well as the Obama administration's use of law enforcement and intelligence agencies for political ends," Grassley wrote in a letter to Comey dated March 28.

Grassley demanded the FBI turn over all its records relating to Steele and the dossier, in addition to "all FBI policies, procedures, and guidelines applicable when the FBI seeks to fund an investigator associated with a political opposition research firm connected to a political candidate, or with any outside entity."

But the most noteworthy thing about Grassley's letter is its focus on McCabe. Grassley noted that McCabe is already under investigation by the FBI's inspector general for playing a top role in the Hillary Clinton email investigation even though McCabe's wife accepted nearly $700,000 in political donations arranged by a close Clinton friend, Virginia Gov. Terry McAuliffe, for her run for state senate in Virginia.

"While Mr. McCabe recused himself from public corruption cases in Virginia…he failed to recuse himself from the Clinton email investigation," Grassley wrote, "despite the appearance of a conflict created by his wife's campaign accepting $700,000 from a close Clinton associate during the investigation."

Now, Grassley wrote, there could be a problem with McCabe's participation in the Trump-Russia probe. If McCabe had a conflict being too close to Clinton, how could he then investigate Trump? A key passage from Grassley's letter:

Mr. McCabe's appearance of a partisan conflict of interest relating to Clinton associates only magnifies the importance of those questions. That is particularly true if Mr. McCabe was involved in approving or establishing the FBI's reported arrangement with Mr. Steele, or if Mr. McCabe vouched for or otherwise relied on the politically-funded dossier in the course of the investigation. Simply put, the American people should know if the FBI's second-in-command relied on Democrat-funded opposition research to justify an investigation of the Republican presidential campaign.

Grassley followed with a dozen questions, all targeted at McCabe. Has McCabe been involved "in any capacity" in investigating alleged collusion between TrumpWorld and Russia? Has McCabe been involved in surveillance or intercepts of any sort in the case? Has McCabe "made any representations to prosecutors or judges" regarding the Steele dossier? Has McCabe had any interactions with Steele himself? Did McCabe brief anyone in the Obama administration on the Trump-Russia investigation? Was McCabe ever authorized by the FBI to speak to the media about the case? Did he ever do so without authorization? Has anyone in the FBI raised questions about McCabe's possible Clinton-Trump conflict of interest? Has any complaint been filed about it? Has anyone at the FBI recommended or requested that McCabe recuse himself from the Russia-Trump investigation?

The March 28 letter is the second Grassley has sent to FBI Director Comey regarding the Trump dossier. Grassley's first letter, on March 6, demanded that the FBI turn over extensive documentation and answer questions about how the bureau came to agree to pay for Steele's services in the midst of a presidential campaign. So far, Grassley has not received the material he seeks.

Look for the chairman to keep pushing. Grassley, a longtime champion of inspectors general in the federal departments, can be a tenacious opponent. Armed with majority control of the Senate Judiciary Committee, he's not likely to give up.