WASHINGTON - Donald Trump may have annoyed the wrong man in Congress.

Sen. Chuck Grassley, the chairman of the Judiciary Committee, has been ramping up an investigation into possible collusion between Russia and Trump's presidential campaign, in addition to the president's dismissal of former FBI Director James Comey.

The plain-spoken Iowa Republican had sharply criticized the administration's initial failure to respond to many lawmakers' requests for information. He also hasn't been shy about other topics, including the use of foul language by the recently dismissed White House communications chief, Anthony Scaramucci.

Grassley's decision to move full speed ahead on Russia, including threatening Trump's son, Donald Trump Jr., and others with subpoenas, will likely force a more public β€” and unpredictable β€” autopsy of topics the administration would rather fade away. Grassley, working with the Judiciary Committee's top Democrat, Dianne Feinstein, is pressing to uncover any attempts to obstruct justice or influence the presidential election.

The senator's approach puzzles some of his Republican colleagues. GOP leaders have preferred to keep the Russia probe limited to the Intelligence Committee, a buttoned-down panel that usually conducts its business behind closed doors.

Colleagues say Grassley remains a loyal Republican who is merely doing what he's done for decades. He has defended Trump on a range of issues, helped install Neil Gorsuch on the Supreme Court, and has lambasted Democratic leaders. He even criticized the media for "hysteria" about the Russia investigations.

"He's a good conservative, but he loves the institution, and he's got that Iowa sense of fairness," said Senator Lindsey Graham, R-S.C., who serves on the Judiciary Committee. "He doesn't go pick a fight, but if you get in a fight with him you've got a worthy opponent. I think the more you stonewall him, the worse it gets."

Earlier this year, some Democrats complained Grassley wasn't doing enough to pursue the Russia matter. Then in June, the committee started delving deeper and the Democrats' complaints turned to praise.

Grassley said he's investigating for a simple reason: Congress has a responsibility to provide oversight.

"This is what Chuck Grassley does," Grassley said last week.

"They may be new to town, but they surely recognize what Chuck Grassley's reputation is. And if they don't know it, they've been told, I bet, a hundred times," he said. "I think I've got a pretty good reputation for being what I call an equal-opportunity overseer."

Grassley said nobody from the White House has asked him to stop or slow his efforts β€” though Trump has repeatedly tweeted about how the Russia probes are unfair and that Republican lawmakers should be standing by him.

Grassley's advocacy has already notched a win, when the White House pledged last month to respond to oversight letters from rank-and-file members in both parties. An earlier legal memo argued that only chairmen were entitled to oversight responses from the administration, prompting a six-page letter of condemnation from Grassley.

"Shutting down oversight requests doesn't drain the swamp, Mr. President. It floods the swamp," Grassley wrote.

"I'd be doing it if Obama was president, and like I told you, back in the Reagan years, I subpoenaed William French Smith," who was then the attorney general, Grassley said.

Grassley, 83, has served in the Senate since 1981 after serving six years in the U.S. House. He was elected to the Iowa state legislature in 1958.

"You want to know what I do?" he said he asks high school students. "Most of you think all we do is pass bills and appropriate money. But that's only half of it. Once we've done that we have a constitutional responsibility to do what the Constitution says. Is the president faithfully executing the laws? Or anybody that works for him?"

If Trump were to try and dissuade him, it wouldn't work, Grassley said. "I treat all presidents the same, Republican or Democrat."

Soon after Trump took office, Grassley tweeted to Trump in hopes the administration would respond to senators' requests for records "so we can do oversight." In June, Grassley wrote a letter blasting a Justice Department opinion arguing that only committee chairmen were authorized to demand responses from the administration.

Grassley's Republican colleagues haven't directly urged him to back off, although some have questioned the probe to reporters, including second-ranking Senate Republican John Cornyn of Texas. The senator said in June it's "wholly inappropriate" for Judiciary to examine possible obstruction of justice because that's the job of Special Counsel Robert Mueller, who is leading a criminal investigation into the Russia matter. "We could end up harming the investigation," Cornyn said.

Grassley brushed off such comments at the time. "They aren't chairman of the committee," he said then. "It's my jurisdiction."

Feinstein said Grassley is "terrific" to work with. "What I know of Grassley is, he's got strong beliefs in what's right and what's wrong," Feinstein said. "He is very honest, very direct, he is without artifice and he's basically a good man. A little gruff sometimes but we all have our ways."

Among the areas they are probing is unregistered lobbying and advocacy efforts by Russians, Ukrainians and others during last year's campaign. That led to Grassley's threat to subpoena the president's son and campaign manager Paul Manafort if they didn't cooperate with a request for testimony and documents relating to a June 2016 meeting with Russians who promised to deliver incriminating information on Democrat Hillary Clinton.

Feinstein said she and Grassley are completing details for what could be two public hearings in late September at which Trump Jr. and Manafort would testify.

"Chuck Grassley is determined, I think, to uncover the truth about any obstruction of justice involving the firing of Jim Comey or other political interference in the investigation of Russian meddling," said Sen. Richard Blumenthal, D-Conn., who serves on the Judiciary Committee.

The Senate Intelligence Committee staff already has interviewed Manafort, and the panel also wants to hear from Trump Jr. and others.

Intelligence Chairman Richard Burr of North Carolina said Grassley's investigation isn't interfering with his probe, which he said has included interviews with more than 70 people.

"I'm not sure that I understand exactly what they're trying to do," Burr said, adding that Judiciary doesn't have access to classified intelligence documents. "They tried to subpoena Paul Manafort; we interviewed him," Burr noted.

Senator Chris Coons, a Democrat on the Judiciary Committee, said it's in the committee's purview to determine whether the president obstructed justice, why Comey was fired, and why Trump has publicly criticized Attorney General Jeff Sessions over his recusal from the Russia probe.

"As long as those are active questions, I think they are well within the jurisdiction of the Judiciary Committee," Coons said.

Graham, who has been critical of Trump, points to the firing of Comey and his subsequent testimony to the Intelligence Committee as a turning point for Grassley.

"Over time, he has gotten much more assertive in terms of the committee's jurisdiction," Graham said of Grassley. "As the facts have unfolded, more and more of the Russian intrigue is in the Judiciary lane. Obstruction of justice, you know, maybe campaign finance laws being violated."

Graham said he's pleased with how Grassley has proceeded.

"In all the things he's done in his very illustrious career, I think this is one of his finest hours," Graham said.