Headlining a rally for Democratic Senate candidate Chris Coons Friday, President Obama warned Delaware voters not to underestimate the electoral challenge posed by Republican Christine O'Donnell this fall, telling the crowd, "I don’t want anybody here taking this for granted."

Obama and Vice President Joe Biden made a joint appearance for Coons Friday in an event that doubled as a fundraiser for the Democratic Senatorial Campaign Committee. 

Even though Coons leads O'Donnell by double digits in recent polling and the president said he thinks Coons "has so far run an extraordinary race," he warned of a tough and unpredictable political environment facing Democrats across the country.

"This is a tough political environment right now," said Obama. "This is a difficult election because we’ve been through an incredibly difficult time as a nation." 

The president implored his party's base to spring into action in November and he attacked Republicans in Washington, who he said made a conscious decision to "sit on the sidelines" as the country's economic problems grew. 

"Their attitude — it was tactical on their part — was that we were climbing out of such a deep hole, they had made such a big mess, that they figured it was going to take some time to repair the economy, longer than any of us would like," he said. "They knew that people would be frustrated. They knew people would be angry." 

O'Donnell responded to the appearance from the president and vice president via Twitter, where she said the campaign visit proves that the contest is closer than polling suggests. 

"We shocked the establishment once, now let's do it again!" O'Donnell tweeted late Friday. 

The president is set to be in Boston Saturday and Ohio on Sunday. Obama also has a planned campaign swing through six states next week, including California, Nevada and Washington state.