Government watchdog Sen Chuck Grassley is waiting to be unleashed.

Blocked by Democratic control of the Senate, the Iowa Republican has had to play a secondary role while investigations he wants to spearhead — such as the probe of Internal Revenue Service targeting of conservative groups — have had to go through his Republican colleagues in the House of Representatives, who are in control and have subpoena power.

But the ranking member on the Senate Judiciary Committee promises a slew of contentious hearings focused on the Justice Department, National Security Agency snooping, Benghazi and others if Republicans win control of the Senate in November.

“[Oversight] is going to be a very big thing as far as I’m concerned. I’m talking about investigations, not just having the attorney general in once a year, or the FBI director in once a year,” Mr. Grassley said in an interview with The Washington Times. “If Republicans are in the majority, I hope we can get the same information this administration says only a chairman is entitled to.”

The Washington bureaucracy, which views Mr. Grassley’s minority party status as a reason not to answer back, have stymied the senator’s effort to hound the Obama administration for information on what he sees as potential abuses of power or mismanagement.

So far this year, Mr. Grassley has pushed the Homeland Security Department for answers about a “hands-off” list that keeps off the no-fly list some individuals with suspected terrorist ties. He has yet to hear back from the agency.

He also has reached out to the inspector general at the Treasury Department to make sure authorities are probing hiring practices of its Financial Crimes Enforcement Network after the division was caught screening job candidates illegally.

The IRS has been another high-profile Grassley target. He said in September that despite the IRS creation of a whistleblower office, the agency has put off processing cases and making financial rewards to workers who report wrongdoing.

He has written President Obama a letter calling on the White House to prohibit federal agencies from designating jobs as “non-critical sensitive” as a way to silence whistleblowers.

“It takes 51 votes to get a bill passed but it only takes one vote to do oversight — in other words, Chuck Grassley making up my mind to do it,” said Mr. Grassley, who said his No. 1 job in Congress is to police the government and defend those who dare to speak out against it.

Mr. Grassley, who spends every weekend in his home state, made a name for himself 30 years ago when he challenged spending at the Defense Department under President Reagan.

Mr. Grassley, then a junior senator and new to the budget committee, wanted to talk with Pentagon analyst Franklin C. “Chuck” Spinney, who wrote a report about what he viewed as wasteful spending.

Defense Secretary Caspar Weinberger dodged the senator’s requests.

“I tried to make arrangements to meet [Mr. Spinney] and Cap Weinberger never set it up so I thought I’m going to go over to the Defense Department and see him myself,” said Mr. Grassley. “When I got over there, Cap Weinberger was gone and they wouldn’t let me talk to Spinney.”

Mr. Grassley returned to his office determined to talk with Mr. Spinney one way or another. He threatened to subpoena Mr. Spinney to get him to testify before the Senate Budget Committee on Defense. Under subpoena pressure, the Pentagon agreed to hold a hearing that Friday afternoon in a remote room to have Mr. Spinney and his superior testify. Mr. Grassley — knowing this was an effort to dodge the press — reconvened the meeting to a larger room and allowed cameras. The Monday that followed, Mr. Spinney’s image was printed on the cover of Time magazine with the headline “U.S. Defense Spending: Are Billions Being Wasted?”

“Sen. Grassley has shown a stronger loyalty to principles than to politics,” said Tom Devine, legal director for the Government Accountability Project, an independent government watchdog. “He’s been the conscience of the Senate — championing the freedom of individuals to challenge the abuses by bureaucracy.”

And Mr. Grassley doesn’t care if he has to get in your face to do it.

Since his days storming the Pentagon, Mr. Grassley has been known to show up at the doorstep if documents are denied or requests are being dodged.

“Chuck is a good, aggressive guy,” said Mort Rosenberg, a fellow at the Constitution Project and a former staffer for the Congressional Research Service who used to work with Mr. Grassley’s staff. “I remember once he was trying to get information about some department head and they were blowing him off and he announced to the press: ‘I’m going to go over there and speak to the secretary and get the information.’ He showed up to the office and they shuffled him to a third-level person in the agency who said ‘no.’ But, eventually he does a good job at annoying people and getting the information he wants.”

Mr. Grassley became the champion for whistleblowers in 1986 when he helped author amendments to the False Claims Act that gave private citizens more power to report and sue government contractors for fraudulent activity. He also co-authored the Whistleblower Protection Act in 1989 as well as the act’s enhancements two years ago.

Mr. Grassley’s whistleblower laws have become the government’s premier anti-fraud tool. Because of Mr. Grassley’s efforts, more than $22 billion have been returned to the Treasury that otherwise would have been lost to fraud, according to an IRS report to Congress penned in 2009.

“The bottom line is: I can have the best staff in the world, four or five or six investigators — and they can do the best job as possible, but this big bureaucracy that we have — you can’t know where all the skeletons are buried,” Mr. Grassley said.

So he relies on whistleblowers — and they come to him.

John Dodson, a special agent with the Bureau of Alcohol, Tobacco, Firearms and Explosives, worked with Mr. Grassley’s staff when he came clean on the Operation Fast and Furious scandal, in which the agency lost track of hundreds of firearms sold to Mexican drug cartels.

In October, Mr. Grassley joined Rep. Darrell E. Issa, California Republican and chairman of the House Oversight and Government Reform Committee, in condemning the ATF for trying to prohibit Mr. Dodson from writing a book about the doomed operation. His account, “The Unarmed Truth: My Fight to Blow the Whistle and Expose Fast and Furious,” was published in December.