US President Barack Obama heads out on his longest campaign swing yet, just as a new poll shows opposition Republicans on the cusp of major gains in next month's congressional elections.

Obama is expected to visit five states in four days in an effort to drum up support for his Democratic Party, which has taken a big hit as a result of the struggling economy and high unemployment.

Two years ago, Obama drew 60,000 to a rally in Oregon, with another 15,000 left outside.

The goal for today's rally in Portland for Democratic gubernatorial candidate John Kitzhaber was only 5000.