BUCHEON, South Korea  The sinking of the South Korean warship Cheonan, apparently by a North Korean torpedo, has provoked an international crisis that has embroiled big powers like the United States and China. But here in South Korea, it has had another effect: buoying the country’s once embattled conservative, pro-American president, Lee Myung-bak.

Soon after taking office two years ago, Mr. Lee appeared at risk of losing public support, as he faced mass demonstrations on the streets of Seoul against the import of United States beef. Now, political experts are talking about the “Cheonan effect,” as polls show that more than half of expected voters approve of the president and his tougher line toward the North.

Nowhere is the current upwelling of popular support more apparent than in polling for the local elections to be held across South Korea on Wednesday. Mr. Lee’s Grand National Party, whose candidates once faced tight races in some districts, now appears poised to sweep the most important races, including hotly contested mayoral elections in Seoul and the nearby port of Incheon.

Kim Moon-soo, the conservative governor of a province outside Seoul, just two weeks ago was in an uphill battle for re-election against a liberal opponent. Now, polls show him with a comfortable 15 percentage point lead.