TORONTO  The Obama administration announced Saturday that it would ask Congress to ratify a long-stalled free-trade agreement with South Korea after the midterm elections in November.

The decision, which risks angering labor unions and their Congressional supporters, was announced as the Group of 20 economies began a two-day summit meeting here, following a smaller meeting by the Group of 8 powers.

After the meeting with the South Korean president, Lee Myung-bak, Mr. Obama’s aides said they would try to resolve lingering issues by the time of the next G-20 leaders’ talks, to be held in Seoul, South Korea, in November, and present the deal to Congress shortly after the November elections.

“We very much welcome and thank President Obama for proposing a date for us to look forward to, and we will work towards that date and objectives,” Mr. Lee said here in Toronto.