When President Barack Obama mentioned the Republican party convention on the campaign trail in Fort Collins, Colorado, and his supporters began to boo his opponents, he paused and went off script.

"Don't boo. Vote. That's the best response. Vote and get some of your best friends to vote," he told the audience at a rally on a campus of Colorado State University.

The tone of urgency was notable. The 2008 version of Obama had no trouble getting people to go to the ballot box during his historic election win. But in 2012 it is a different game. Battered by the realities of four years in office and with a party base that often believes he has let them down, Obama is facing an enthusiasm gap as he prepares to rally Democrats at their nomination convention in Charlotte, North Carolina.

It will not be an easy task and his speech on Thursday night will be a test of whether Obama the president can rekindle the sort of emotion that saw Obama the candidate run one of the most remarkable campaigns of modern American history. "A lot of people feel that it has been politics as usual and there has been no fundamental change. It is a real problem for him," said Professor Shaun Bowler, a political scientist at the University of California at Riverside.

That enthusiasm gap in the fight against Republican challenger Mitt Romney can be read in poll numbers. At this point in 2008, 63% of Americans viewed Obama favourably. Now it is 53%. It can also be seen at the rallies. Though Obama can still draw a crowd, the numbers, and the venues, are smaller than four years ago.