A special prosecutor will be needed to investigate the IRS' targeting of conservative groups, Sen. Rob Portman told Newsmax in an exclusive interview, because there are limits to what Congress can achieve on its own.

The Ohio Republican said that while he believes the congressional investigation should continue for a while longer, it's unlikely that lawmakers will be able to extract the information they need from the administration to get to the bottom of the issue.

"We ought to let this congressional process play out for a while longer and see if we can't get the administration to provide the information that the American people deserve. As I have said really for weeks now, I am skeptical that they will do that, and therefore a special counsel will end up being necessary," he told Newsmax TV.

Portman said part of the problem is that so far the White House has failed to produce the information requested by lawmakers.

"The administration is not coming forward with the information that we've requested. When we wrote letters about this issue back in 2012, they responded saying everything's fine, there's no targeting going on, and even now the answer to our questions are not the kind of comprehensive answers we need to get to the bottom of it."

He added, "We need to get them to come clean, the American people need to know what actually happened, who was responsible, why it happened, so that we can get to the bottom of it and keep it from ever happening again."

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Portman said a special counsel would have the power to force information from the administration by taking forward a criminal probe, which includes the authority to put in place a grand jury and be able to give witnesses immunity to get them to testify.

"This is a very serious matter because it has to do with arguably the department who has the most power over our lives, power to tax. Chief Justice [John] Marshall once said it has the power to destroy.

"This is an agency that affects all of us in a very fundamental way, and ultimately, this is about not just political targeting of one group or another, but it's about a lack of fairness in the application in the administration of our tax laws, which we must get to the bottom of and we must stop."

Portman also said that if IRS employees are found to have violated the constitutional rights of a taxpayer, the law dictates they should face termination, not administrative leave, as is the case with a number of the IRS employees involved in the current scandal.

"I would hope [the IRS] would take action that anybody would take in the private sector, if you have an employee who violated the basic tenets of a fair and even-handed administration of tax laws."