Searching for help, President Barack Obama lobbied the skeptical leaders of Russia and China on Saturday for support in keeping Iran from becoming a nuclear-armed menace to the world, hoping to yield a "common response" to a deepening problem.

Yet Obama’s talk of solidarity with Russian President Dmitry Medvedev and Chinese President Hu Jintao was not publicly echoed by either man as Iran moved anew to the fore of the international stage — and to the front of the fierce U.S. presidential race.

The United States’ vast worries about Iran grew starker with a report last week by the U.N. atomic agency that asserted in the strongest terms yet Iran is conducting secret work with the sole intent of developing nuclear arms. The U.S. claims a nuclear-armed Iran could set off an arms race among rival states and directly threaten Israel.

Russia and China remain a roadblock to the United States in its push to tighten international sanctions on Iran. Both are veto-wielding members of the U.N. Security Council and have shown no sign the new report will change their stand.

Meeting with Medvedev on the sidelines of the Asia-Pacific summit here, Obama said the two "reaffirmed our intention to work to shape a common response" on Iran.

Shortly after, Obama joined Hu, in a run of back-to-back diplomacy with the heads of two allies that hold complicated and at times divisive relations with the United States.

Speaking to reporters before he talked to Hu, Obama said that he and the Chinese leader want to ensure that Iran abides by "international rules and norms."

Obama’s comments were broad enough to portray a united front without yielding any clear indication of progress. Medvedev, for his part, was largely silent on Iran during his remarks, merely acknowledging that the subject was discussed. Hu did not mention Iran at all.

As the president held forth on the world stage in the state where he was born and grew up, Republicans vying to compete against Obama for the presidency unleashed withering criticism in a debate in South Carolina. It was a rare moment in which foreign policy garnered attention in a campaign dominated by the flagging U.S. economy.

"If we re-elect Barack Obama, Iran will have a nuclear weapon. And if you elect Mitt Romney, Iran will not have a nuclear weapon," said Romney, the former Massachusetts governor. Minnesota Rep. Michele Bachmann warned that Iran’s attempt to develop a nuclear weapon is setting the table "for worldwide nuclear war against Israel."