WASHINGTON (AP) — Congress is stepping up pressure on the White House to confront Russia over allegations that it is cheating on a key nuclear arms treaty — a faceoff that could further strain U.S.-Moscow relations and dampen President Barack Obama’s hopes to add deeper cuts in nuclear arsenals to his legacy.

Butting heads with Russian President Vladimir Putin over compliance with a 26-year-old treaty to eliminate an entire class of nuclear weapons is not something that fits into Obama’s “reset” with Russia, which already was stalled after Russia granted asylum to National Security Agency leaker Edward Snowden and annexed Ukraine’s Crimean Peninsula. But the issue has been simmering for a few years and Republicans on Capitol Hill want Obama to address it head-on.