"There is no doubt this is a difficult election. This is a tough political environment," Obama told voters in Delaware, where Democrat Chris Coons holds a solid lead in the polls over tea party-backed Republican Christine O'Donnell.
Speaking on the stage of Wilmington's ornate Grand Opera House, Obama said Coons, a county executive, shares the same values as Vice President Joe Biden, who held the same Senate seat and joined Obama at Friday's rally.
With fewer than three weeks until the Nov. 2 election, Obama is on a campaign blitz, traveling coast to coast trying to raise money and generate enthusiasm for Democratic candidates trying to hold the party's majorities in the House and Senate. He'll travel to Massachusetts and Ohio this weekend.
Obama and Biden's joint stop to campaign for a candidate with an overwhelming lead in the polls shows just how cautious Democrats are being, having learned from Republican Sen. Scott Brown's January victory in Massachusetts' special election that no state is safe.
Democrats are also hoping Obama's stop in Delaware will boost the party's candidates in neighboring Pennsylvania, particularly in Philadelphia, which shares a media market with Wilmington. The prospects of Democratic Senate candidate Joe Sestak could hinge on turnout in cities like Philadelphia.
Obama's message to voters: while the economy may still be struggling to rebound from the recession and middle class families are still hurting, the GOP only wants to return to the policies he says led to the economic crisis. And he has tough talk for Democratic voters whose enthusiasm has waned since the 2008 presidential election, and who party officials worry won't show up at the polls in November.
"Don't let them take this country backward because you didn't care enough to fight for it," Obama told the crowd in Delaware.
Though Obama never mentioned O'Donnell by name, her surprise victory in the Republican primary has turned the national spotlight on the Delaware Senate race, exposing deep divisions between tea party supporters and the GOP establishment.
Republican leaders in Washington and Delaware criticized O'Donnell in unusually strong terms before her win, accusing her of lying about her education and record, leaving a trail of debt that included tax liens and a default on her mortgage, and using campaign finances for personal expenses.
Tom Ross, the state GOP chairman, described O'Donnell as a liar and fraud who couldn't get elected dogcatcher.
O'Donnell used the social networking site Twitter to criticize Obama and Biden's visit, saying it shows Coons was trying to use Friday's rally to divert from his record.
Obama praised Coons for cutting spending and balancing the budget in New Castle County, the state's largest. He didn't mention that Coons also has pushed through three property tax increases since being elected in 2004, including a 25 percent hike for 2010. Coons has said the increases were made necessary by falling tax revenues in the sour economy and that he would have had to lay office police officers and other critical staff without new funding.
Associated Press writer Julie Pace in Washington contributed to this report.