U.S. Democrats officially nominated President Barack Obama as their candidate for the next presidency in a roll call vote running into early hours of Thursday.

Obama passed the threshold of required 2,777 delegates halfway through the roll call vote at Democrats' national convention in Charlotte, North Carolina.

Obama will formally accept the party's nomination on Thursday night and deliver his prime time speech.

The focus of his acceptance speech will be a "forward-going" plan, said the Obama campaign.

The bad weather forecast has forced Democrats to move Obama's big moment speech indoors, the Democratic National Convention Committee announced on Wednesday morning.

The venue for the speech, which was originally planned in Bank of America Stadium, would be changed to Time Warner Cable Arena, the site of the 2012 Democratic National Convention in the first two days.

As it was planned initially by the organizer, the convention was to be held on Tuesday and Wednesday in Time Warner Cable Arena, a downtown sports and entertainment arena with maximum capacity of 20,000 seats, and then to be moved to the open-air Bank of America Stadium, which could accomodate up to 70,000 people.

Democrats hope the three-day convention would generate a renewed enthusiasm for Obama's re-election bid and help him win the battleground of North Carolina.

Organizers of the convention said nearly 6,000 delegates would attend the three-day convention, among whom half would be woman voters. More Latinos were also expected to appear at the venues.

Recent polls showed that Obama's healthcare reform and immigration policy give him an edge over his Republican rival Mitt Romney among woman and Latino voters in the swing states, including North Carolina.

Right now Obama and Romney are deadlocked in nationwide voter support. However, a recent Washington Post-ABC News poll showed Obama has to do more to thrill the base during the Democratic National Convention.

Currently, 48 percent of Obama's supporters are "very enthusiastic" about his candidacy, while 42 percent of Romney's backers feel that way about the Republican campaign, according to the poll. In July, Obama led by 13 points and even by 25 points in May.

A recent Pew poll found that about four in ten Americans are interested in following what happens at the Democratic National Convention in Charlotte, down from 59 percent four years ago when Obama became the party's presidential nominee for the first time.