Tonight's speakers, Castro and the first lady were picked to target two decisive Democratic voting blocks, women and Hispanics.
Castro credits Obama with saving the country from financial ruin four years ago, a direct response to Romney's latest line of attack, asking voters if they're better off today than they were four years ago.
"Four years ago, America stood on the brink of a Depression. Despite incredible odds and united Republican opposition, our president took action. And now we've seen 4.5 million new jobs," Castro said.
Castro said voters have to choose between a country "where the middle class pays more, so that millionaires can pay less... Or a country where everybody pays their fair share."
Despite the rhetoric and applause, Democrats on the floor conceded the convention lacked the electricity of four years ago when Obama made history as the first black man to nominated for president and promised hope and change.
"It feels different," said Kooch Jacobus, a delegate from Albuquerque, N.M., who was at the 2008 convention. "Then it was hope and change and now reality has hit us. "
Jacobus said that feeling of 2008 was something she hoped would have been recreated, but said that it has not yet.
Another delegate pointed out delegates on their iphones, ipads and shook their head at observing one delegate sleeping. The delegate says that would never happen in 2008, even on a first night of speeches, because people were much more fired up then.
ABC News' Sunlen Miller contributed to the report