Former US President Bill Clinton has delivered a prime-time defence of Barack Obama, on the second night of the Democratic convention.
Taking the stage in Charlotte, North Carolina, he formally proposed President Obama as Democratic nominee.
He launched a full-throated defence of Mr Obama's policies, saying his economic record had been a success.
Earlier, organisers moved Mr Obama's acceptance speech from an outdoor stadium, citing poor weather forecasts.
They moved Thursday's speech from Charlotte's 74,000-seat Bank of America stadium to the 20,000 capacity indoor arena where the rest of the convention is taking place.
But the capacity of the Time Warner Cable Arena quickly became a focus of attention as hundreds of people were reportedly locked out of the building because of capacity issues during the evening session.
'On the job'
Mr Clinton offered a lengthy defence of Mr Obama's record, attacking Republicans for blocking further progress on the economic recovery and getting deep into the detail of policy debates.
"In order to look like an acceptable, moderate alternative to President Obama, they couldn't say much about the ideas they have offered over the last two years," he said, referring to the Republican convention in Florida a week ago.
Reminding the crowd that Senate Republican leader Mitch McConnell had revealed that their number one priority was to get Mr Obama out of office, he declared: "We're going to keep President Obama on the job," he said.
Mr Clinton argued that Mr Obama's economic policies on taking office had prevented further collapse and begun the recovery, but said he knew that many Americans were not feeling it yet.
He compared Mr Obama's experience to his own first term in office, when "our policies were working and the economy was growing but most people didn't feel it yet".
"No president - not me or any of my predecessors - could have repaired all the  damage in just four years," he said. "But conditions are improving and if you'll renew the president's contract you will feel it."
After staying away on Tuesday when his wife Michelle spoke, President Obama travelled to the convention centre for Mr Clinton's address.
Wednesday night's speech is being seen as the high point of a revitalised relationship between the two presidents and as an attempt to boost Mr Obama's appeal with white working-class voters.
Polls show these traditional Democratic voters are wary of Mr Obama, but Mr Clinton has a strong record in winning their support.
The pair have previously sparred, most notably during the 2008 primaries when Mr Clinton supported his wife Hillary's bid for the nomination, and they are known not to have a close personal bond.
Democrats hope the former president, who oversaw a booming economy and balanced budgets during two terms in the White House, can help dispel voter perceptions that Mr Romney would do a better job than Mr Obama.
Meanwhile, Chicago Mayor Rahm Emanuel has been drafted to work with Priorities USA, a political fundraising committee supporting Mr Obama, the Washington Post reported.
Mr Emanuel, who addressed the convention on Tuesday, has a reputation as a formidable fundraiser, a boon to Democrats who have not attracted as much money from donors as Republicans in recent months.
On Tuesday, Michelle Obama's speech was one of the few that did not mention Mitt Romney, the former Massachusetts governor, by name.
She told a crowd of supporters that her husband's experiences and struggles had guided his decisions as president.
Meanwhile, Mr Romney is spending the week away from the campaign trail, preparing for a series of debates with Mr Obama.
But vice-presidential running mate Paul Ryan kept up the attack, telling Fox News: "What you did not hear [at the Democratic convention] is that people are better off than they were four years ago."
Republicans are focusing their argument on the fragile US economy, which has had 42 months of unemployment surpassing 8% - the longest such period since the end of World War II.
No president since the Great Depression has been re-elected with joblessness so high.
Source: BBC News