President Obama rallies Democrats

US President Barack Obama kicked off a busy week of campaigning by attempting to energise Democrat supporters ahead of midterm elections in November.

Republicans are expected to make big gains in the congressional ballots and President Obama must limit the damage by mobilisng a coalition of young voters and the Democratic party's progressive base that won him the White House in 2008.

"I think we are going to win, but you have to prove them wrong," Obama told a cheering crowd of thousands at an outdoor rally. "It's up to you to show the pundits that you care too much about this country to let it fall backwards, that you are going to keep us moving forward, that you are ready to fight for the future," he said.

The Pennsylvania crowd was estimated at over 18,000 by city officials, the rally organizers said.

In a packed schedule Obama will travel to Florida on Monday to campaign for Democrats, take part in two townhall style events designed to address ordinary Americans during the week, visit Delaware on Friday, and stump for his party again in Massachusetts on Saturday.

He will also host a meeting at the White House on Monday morning to showcase his push for additional infrastructure spending to boost hiring and communicate that he understands the fear of American households over unemployment.

"Philly, let's prove them wrong. Let's show Washington one more time change doesn't come from the top, it comes from the bottom," he said, urging Democrats to recapture the enthusiasm that won him the presidency in 2008. "It doesn't come from millions of dollars of ads, it comes because people are out there knocking on doors, making phone calls, going into the beauty shops, going into the barber shops," he said.