President Barack Obama told Russia's leader Monday that he would have more flexibility after the November election to deal with the contentious issue of missile defense, a candid assessment of political reality that was picked up by a microphone without either leader apparently knowing.

Obama's Republican opponents pounced on the comment, saying the president has a hidden agenda that could include concessions to the Russians if he is re-elected this fall.

"This is my last election," Obama is heard telling outgoing Russian President Dmitry Medvedev. "After my election, I have more flexibility."

Medvedev replied in English, according to a tape by ABC News: "I understand. I will transmit this information to Vladimir," an apparent reference to incoming President Vladmir Putin.

Obama and Medvedev did not intend for their comments, made during a meeting in Seoul, South Korea, to be made public.

Once they were, the White House said Obama's words reflected the reality that domestic political concerns in both the U.S. and Russia this year would make it difficult to fully address their long-standing differences over the contentious issue of missile defense. Obama, should he win re-election, would not have to face voters again.

"Since 2012 is an election year in both countries, with an election and leadership transition in Russia and an election in the United States, it is clearly not a year in which we are going to achieve a breakthrough," White House deputy national security adviser Ben Rhodes said.

Obama's candid remarks Monday illustrated the political constraints that hem in any president who is running for re-election and dealing with a congressional chamber — in this case, the House — controlled by the rival party. Republicans have fought Obama fiercely on health care, taxes and other issues. They are eager to deny him any political victories in a season in which they feel the White House is within reach, although Obama's remarks suggested he feels good about his re-election prospects.

Even if Obama was confiding a political reality in a supposedly private moment, the comments gave the GOP new openings to question his sincerity and long-range plans.

Mitt Romney, the leading Republican contender to face Obama this fall, told a San Diego audience the unguarded comments were "an alarming and troubling development."

"This is no time for our president to be pulling his punches with the American people, and not telling us what he's intending to do with regards to our missile defense system, with regards to our military might and with regards to our commitment to Israel," Romney said.