For the second time in a week, Obama told thousands of young people attending a rally to come out and vote in this fall's mid-term elections to preserve Democratic majorities in Congress that could help the president move forward on his agenda.

He also struck at Republicans, who he said were hoping that voters had amnesia and would forget about GOP policies that brought the country's economy to the point of collapse.

"They say that there is an 'enthusiasm gap,' and that the same Republicans and the same policies that left our economy in a shambles and the middle class struggling year after year -- that those folks might all ride back into power. That's the conventional wisdom in Washington," Obama said at a fundraiser/concert for young Democratic activists at Washington's D.A.R. Constitution Hall. Hip hop artist B.o.B. performed before Obama took the stage. 

"We cannot let that happen. We cannot sit this out. We can’t let this country fall backwards," Obama told the crowd. "The stakes are too high. We have to move this country forward for you and your future," he said. "So there better not be an enthusiasm gap, people. Not now. Not this time."

Over the past week, Obama has tried to reenergize young voters, one of the core constituencies that drove him to victory in 2008.

The president on Tuesday held a rally with national musical acts at the home of the University of Wisconsin, Madison, which was the site of one of the largest rallies of his presidential campaign.

Democrats sent out multiple releases this week touting the the raucous crowd of 26,000 who turned out to see Obama speak.

Polls suggest Republican voters are more motivated to come out and support their candidates this fall than Democrats.

Obama said Thursday that his political opponents are hoping that voters forgot how their policies helped send the U.S. economy into a tailspin. 

"The other side would like you to believe that this election is a referendum on me, or on the economy -- on anything but them," he said. "They are counting on amnesia. They’re counting that folks don't remember them. But make no mistake. This election is a choice. And the choice could not be clearer."

Republicans are expected to make large gains on the Democrats' majorities in Congress, with some political observers predicting that the GOP will take back the House after losing control of it in 2006. 

Those handicappers point to the large gap in voter enthusiasm showed in national polling data and the slow pace of the economic recovery under Obama.