WASHINGTON (Reuters) - A reported U.S. plan to keep some suspected terrorists imprisoned for a lifetime even if the government lacks evidence to charge them in courts was swiftly condemned on Sunday as a "bad idea" by a leading Republican senator.

The Pentagon and the CIA have asked the White House to decide on a more permanent approach for those it was unwilling to set free or turn over to U.S. or foreign courts, the Washington Post said in a report that cited intelligence, defense and diplomatic officials.

Some detentions could potentially last a lifetime, the newspaper said.

Influential senators denounced the idea as probably unconstitutional.

"It's a bad idea. So we ought to get over it and we ought to have a very careful, constitutional look at this," Republican Sen. Richard Lugar of Indiana, chairman of the Senate Foreign Relations Committee said on "Fox News Sunday."

Sen. Carl Levin of Michigan, senior Democrat on the Armed Services Committee, cited earlier U.S. Supreme Court decisions. "There must be some modicum, some semblance of due process ... if you're going to detain people, whether it's for life or whether it's for years," Levin said, also on Fox.

The White House did not immediately respond to a request for comment. The State Department declined comment and a Pentagon spokeswoman, Lt. Col. Ellen Krenke of the Air Force, had no information on the reported plan.

As part of a solution, the Defense Department, which holds 500 prisoners at Guantanamo Bay, plans to ask the U.S. Congress for $25 million to build a 200-bed prison to hold detainees who are unlikely to ever go through a military tribunal for lack of evidence, defense officials told the Washington Post.

The new prison, dubbed Camp 6, would allow inmates more comfort and freedom than they have now, and would be designed for prisoners the government believes have no more intelligence to share, the newspaper said.

"It would be modeled on a U.S. prison and would allow socializing among inmates," the paper said.

"Since global war on terror is a long-term effort, it makes sense for us to be looking at solutions for long-term problems," Bryan Whitman, a Pentagon spokesman, was quoted as saying. "This has been evolutionary, but we are at a point in time where we have to say, 'How do you deal with them in the long term?"'

The Post said the outcome of a review under way would also affect those expected to be captured in the course of future counterterrorism operations.

One proposal would transfer large numbers of Afghan, Saudi and Yemeni detainees from the U.S. military's Guantanamo Bay, Cuba, detention center into new U.S.-built prisons in their home countries, it said.

The prisons would be operated by those countries, but the State Department, where this idea originated, would ask them to abide by recognized human rights standards and would monitor compliance, a senior administration official was quoted as saying.