ATLANTA (Reuters) - After a brutal election loss in November, Democrats will choose a new leader on Saturday to begin the daunting task of rebuilding the party and heading the political opposition to Republican President Donald Trump.

The 447 members of the Democratic National Committee, the administrative and fundraising arm of the party, will gather in Atlanta to pick a new chairman in one of the most crowded and competitive party leadership elections in decades.

The stakes are high for a party still struggling to recover from the surprising Nov. 8 loss of Democratic presidential candidate Hillary Clinton and anxious to channel the growing grassroots resistance to Trump into political support for Democrats at all levels of government across the country.

Seven candidates are vying for the chairmanship, led by former Labor Secretary Tom Perez, a favourite of former Obama administration officials, and U.S. Representative Keith Ellison of Minnesota, backed by liberal leaders such as U.S. Senators Bernie Sanders of Vermont and Elizabeth Warren of Massachusetts. Perez is the son of Dominican immigrants, while Ellison was the first Muslim elected to the U.S. Congress.

The clash between candidates representing the establishment and progressive wings of the party echoes the bitter 2016 primary between Clinton and Sanders, a rift Democrats hope to put behind them as they turn their focus to fighting Trump.

The election also offers the DNC a fresh start after last year's forced resignation of chairwoman Debbie Wasserman Schultz, who stepped aside after the release of hacked emails that appeared to show DNC officials trying to help Clinton defeat Sanders in the primaries.

Other contenders are Pete Buttigieg, the mayor of South Bend, Indiana, Idaho Democratic Party Executive Director Sally Boynton Brown, election lawyer Peter Peckarsky, and activists Jehmu Greene and Sam Ronan.

All of the candidates have pledged to focus on a bottom-up reconstruction of the party, which has lost hundreds of statehouse seats under Obama and faces an uphill task in trying to reclaim majorities in Congress in next year's midterm elections. Ellison has pledged to start building Democratic organizations in each U.S. county.

"We can actually help Democrats win all over the country so that we can get rid of Donald Trump," Ellison said at a candidates' debate on CNN earlier this week. "That means we focus on turnout, and that is how we succeed."

Perez also has promised to confront Trump as Democrats try to harness the energy of the anti-Trump resistance.

"The Democratic Party needs to take the fight to Donald Trump. When we lead with our values, when we lead with our conviction, that's how we succeed," Perez said at the debate.

But Buttigieg warned Democrats it is "not all about Donald Trump," calling him a computer virus in the political system.

"We can't let him dominate our imagination, because it's our values and our candidates that matter most," he said.

Perez and Ellison are considered to be running neck-and-neck in the race to win a majority of the 447 DNC members, who include state party officials, donors and activists from all 50 states.

But if no candidate wins a majority on the first ballot - a strong possibility given the large field - additional rounds of voting will be held. After two rounds, the candidate with the lowest vote total is eliminated.

That could lead to plenty of deal-making and turn some of the contenders into potential kingmakers.

(Writing by John Whitesides; Editing by Mary Milliken)