(CNN) Sen. Chuck Grassley is asking the FBI if it warned Donald Trump that Paul Manafort, whom he brought in as campaign chairman in 2016, was being watched by the agency because of his dealings with pro-Kremlin figures.

And if there was no such FBI warning, Grassley says he wants to know why.

The chairman of the Senate judiciary committee made the request in a letter to FBI Director Christopher Wray on Wednesday.

The FBI declined to comment.

"I write to inquire about whether the FBI ever provided the Trump campaign with a defensive briefing or other warning regarding attempts to infiltrate the campaign by people connected with, or compromised by, Russian intelligence," Grassley wrote.

CNN reported this week that the FBI twice has obtained court permission, through so-called FISA warrants, to wiretap Manafort. One of those warrants was issued before Manafort joined the Trump campaign.

"This raises the question of whether the FBI ever alerted Mr. Trump to the FBI's counter-intelligence concerns regarding his campaign manager and others associated with the campaign -- so that he could take defensive action to prevent the campaign from being infiltrated," Grassley wrote.

"If the FBI did provide a defensive briefing or similar warning to the campaign, then that would raise important questions about how the Trump campaign responded," Grassley wrote. "On the other hand, if the FBI did not alert the campaign, then that would raise serious questions about what factors contributed to its decision and why it appears to have been handled differently in a very similar circumstance involving a previous campaign."

Grassley asked Wray to respond to his letter by October 4.

In the process, Grassley infuriated a GOP Senate colleague -- John McCain of Arizona -- by dragging up McCain's past association with consultants who did pro-Putin work. A source who had contact with the senator Thursday described him as "hot" about the letter's phrasing.

The Grassley letter included this line: "A recent report quoted an anonymous US counterintelligence official who had been involved as saying: 'Before there was Trump, there were concerns about some of the same people being around McCain about 10 years ago, and we alerted his team to those concerns and they appeared to take some defensive action.'"

The Grassley letter also cites media interviews in which longtime McCain political adviser John Weaver suggests the McCain campaign received a warning from intelligence officials.

McCain's 2008 campaign manager, Rick Davis, was a longtime business partner of Manafort. And when they were working for pro-Kremlin clients, McCain twice met with a Russian oligarch named Oleg Deripaska. His name is back in the news this week because of Washington Post reporting that Manafort, while on the Trump campaign payroll, offered a private campaign briefing to Deripaska.

There is nothing new here about McCain's conduct; details of the Deripaska meetings were reported during his 2008 presidential campaign. But the senator is fiercely protective of his reputations as an anti-Putin hawk, and bristles at any suggestion his personal integrity has been compromised.

He also is in a grueling battle with brain cancer, and a friend tells CNN: "You know how he is about that kind of (expletive) anyway, but the timing sucks." The separate source who described McCain as "hot" also made note of the timing.

"He is stuck here, having treatments, and not in a good place," this source said, describing McCain's reaction to the Grassley as "quite volcanic."

McCain spokeswoman Julie Tarallo said: "Neither Senator McCain nor anyone on his staff recalls receiving such warnings from the intelligence community. Senator McCain had two interactions with Mr. Deripaska in 2006, and both were social occasions and entirely incidental."

She added: "No member of Congress has done more to push back on Russian aggression, human rights abuses, and corruption than Senator John McCain. Any suggestion to the contrary is clearly intended to distract from the serious ongoing investigations into Russia's interference in our election system."

Weaver, a top strategist on the 2008 campaign, told CNN that Grassley's letter is wrong.

"At no time have I ever suggested, as it did not happen, that intelligence officials contacted the campaign about Manafort/Davis," Weaver said.

"Shame on Grassley," he added.

This story has been updated.