Barack Obama on campaign trail as Democrats begin convention

US President Barack Obama hit the campaign trail Tuesday as his Democratic Party prepares to nominate him as its candidate to seek four more years as president in November elections, DPA reported.

"I know that whatever I say here today, it's going to be at best a distant second to the speech you will hear tonight from the star of the Obama family, Michelle Obama," the president told a rally in Norfolk, Virginia.

First Lady Michelle Obama will make the case at the convention's opening night for her husband's relection before thousands of the party faithful gathered in the southern US city for the centre-left party's three-day gathering.

"I'm going to be at home, and I'm going to be watching it with our girls," Obama said of his wife's speech. "And I am going to try not to let them see their daddy cry."

With unemployment stuck above 8 per cent and second-quarter growth at an annual rate of 1.7 per cent, Democrats were labouring this week in Charlotte to rediscover the same enthusiasm that swept Obama to power four years ago. Obama will arrive in Charlotte Wednesday and will give his acceptance speech Thursday.

The party conventions give candidates coveted time to make their case to the American public during evening television viewing hours. The centre-right Republicans last week formally named Mitt Romney their candidate.

Democratic campaign spokeswoman Jen Psaki said Michelle Obama would give "a personal and passionate speech" about what drives her husband and his values. Romney's wife, Ann, gave a similar address in a bid to show the personal side of her husband last week.

Also on the programme Tuesday is San Antonio Mayor Julian Castro, the first Hispanic to give a keynote address at the party convention. The key speaking slot pegs him as a rising star within the party that is eager to claim support from the large voting bloc.

The Republicans have focussed on the economic data to tell voters it is time to make a change.

"Are you better off now than you were four years ago?" vice presidential candidate Paul Ryan asked at a rally Tuesday.

Obama however has focussed on the contrast between his policies and his opponents. "We're not moving backward, we're moving forward," he said painting his policies as best for the middle class.

"On issue after issue, governor Romney and Congressman Ryan want to take us backward," he said in Norfolk. "But the story of America is not about going backward it's about going forward."