Obama was overheard telling Russian President Dmitry Medvedev during a nuclear security summit in Seoul this week that he'd have "more flexibility" to tackle missile defense issues "after my election" in November.
Washington and NATO say the missile shield would protect Western nations from missile attacks from possible nuclear powers such as North Korea and Iran. Moscow says the shield could weaken its own nuclear capabilities and proposes collaborating on a joint system.
House Speaker John Boehner, R-Ohio, in a letter to Obama expressed concern about the remarks, caught on an open microphone after a news conference.
Boehner said he had concerns over Moscow's support for the Iranian, Syrian and North Korean governments, all countries suspected of pursuing a nuclear weapons program.
"It is troubling that you would suggest to Russian leaders that their reckless ambition would be rewarded with greater 'flexibility' on our missile defense program after the upcoming election," Boehner states.
The Obama administration defended that statements by saying it would be difficult to have a candid conversation about a complex issue like missile defense during a presidential campaign season.