Facing a close election and Republican attacks that they have made things worse while in power, President Barack Obama and Democrats seek to emphasize what has been achieved and additional steps to bolster the middle class at their three-day national convention that begins Tuesday.

The political conclave that will formally nominate Obama for a second term serves as a response to last week's Republican convention that nominated Mitt Romney as the GOP challenger in November.

Democrats offered a glimpse of issues expected to play a prominent role in this week's events, releasing their party platform late Monday. It focuses on improving the economic situation for middle-class Americans, a central theme of Obama's campaign and an issue the party hopes will win votes come November. It also contains language endorsing same-sex marriage for the first time, a move that brings the party's official stance in line with that of the president, who said for the first time in May that he supports marriage rights for gays and lesbians.

"Over the course of the week, you'll hear a very different tone than the one that you heard last week in Tampa, which was really essentially one nonstop series of attacks on President Obama," Rep. Debbie Wasserman Schultz, the chairwoman of the Democratic National Committee, told CNN on Tuesday.

"We're going to lay out the case for moving the economy forward. President Obama and speakers throughout the week will talk about and have an honest conversation about where we were when he first took office and where we are now after four years of his policies and 29 straight months of job growth in the private sector. And that we need to continue to move forward and we've got a ways to go."

First lady Michelle Obama will address the convention Tuesday night, and former President Bill Clinton headlines the second night before Obama concludes it with his nationally televised address Thursday night.