WASHINGTON - Criticism of 47 Republican senators' letter to Iranian leaders escalated Friday, and one of the lawmakers expressed misgivings about writing directly to an adversary to raise doubts about President Obama's nuclear negotiations.

Several newspapers that had endorsed the senators' elections were harshly critical. A handful of conservative commentators and former GOP aides joined liberals in calling the letter ill-advised.

Republican Sen. Ron Johnson of Wisconsin, who may face a tough reelection next year, defended the letter, but added, "If there was any regret, tactically, it probably would have been better just to have it be an open letter addressed to no one."

Another signer, Sen. Pat Roberts of Kansas, expressed similar thoughts. The letter "could have been addressed to other folks and gotten the message out," he said. "But I think the message is more important than who we send it to."

All but seven of the Senate's Republicans signed the letter, but no Democrats did. The letter warns Iran's leaders that any negotiated deal on their nuclear program could expire when Obama left office.

Democrats and some academics say the letter undermines Obama's - and future presidents' - ability to set foreign policy.

Republicans defended the letter, saying they must take dramatic steps to demand a voice in negotiations because they fear Obama will be too soft on Iran. Some of the 47 senators, however, are taking heat back home from editorial pages that have supported them.

Newspapers in Ohio, New Hampshire, Illinois, and Utah criticized the letter. "The magnitude of this disgraceful decision," the Cleveland Plain Dealer said, "shows the degree to which partisanship has gobbled up rationality on foreign policy."

Some of the seven GOP senators who didn't sign questioned colleagues' actions. "I just didn't feel that it was appropriate or productive at this point," said Sen. Jeff Flake of Arizona.

Richard Haass, head of the Council on Foreign Relations and an aide to both Bushes, said partisan overtures such as the letter made the world more uncertain, dangerous, and disorderly.