Philadelphia - The Democratic Party kicks off its national convention on Monday to anoint Hillary Clinton its presidential nominee, as a row over leaked emails showing party leaders sought to undermine campaign rival Bernie Sanders threatened to upstage the gathering.Sanders lost the primary race but he has endorsed his bitter rival, and in a show of unity he has been offered a prime speaking slot on day one of the four-day confab.Sanders will stress that the party mission is to elect Clinton in November and prevent Republican Donald Trump from becoming the 45th president of the United States.The brash billionaire, who was formally nominated the Republican Party's flag bearer in Cleveland last week, topped Clinton 48 to 45% in a two-way match-up CNN poll out Monday. The figure represents a drastic six-point convention bounce.While the former secretary of state is set to make history as the first female flag bearer of any major American political party, the process has fallen under a cloud that Trump has been all too eager to exploit.Anti-secrecy website WikiLeaks at the weekend released nearly 20 000 emails from between January 2015 and May 2016, gleaned by hackers who apparently raided the accounts of seven Democratic National Committee leaders.At least two of the messages showed senior committee members were keen to undermine the Sanders campaign by seeking to raise questions about Sanders's faith and heritage.Amid efforts to draw a line under the damage that threatened to revive tensions with Sanders followers, the Democratic Party's chairwoman Debbie Wasserman Schultz abruptly announced her resignation, effective at the end of the convention.After a hard-fought primary campaign, the party had been heading to the convention seeming far more unified than the Republicans, whose fissures were laid bare in Cleveland last week.Now the Democrats are struggling with the fallout from a scandal that threatened to mushroom into a major crisis just as the party was supposed to coalesce around its nominee.Total meltdown"The Democrats are in a total meltdown," Trump taunted on Twitter. "E-mails say the rigged system is alive & well!"Trump has long sought to scoop up disaffected voters who feel Sanders - a self-described democratic socialist initially dismissed as a fringe candidate - was denied a fair shot at the nomination.Wasserman Schultz's announcement came after Sanders on Sunday repeated calls for her to go, with her leadership already under fire and her impartiality called into question by the leaks.Shortly after she resigned, Sanders said in a statement that Wasserman Schultz "has made the right decision for the future of the Democratic Party."He called for new leadership that would "always remain impartial in the presidential nominating process, something which did not occur in the 2016 race."Wasserman Schultz said she would still open and close the convention, but the move carries risks, especially if she is booed when she takes the stage.Despite the swirling political chaos, Sanders made clear he would not make an insurgent bid for the nomination."We've got to elect secretary Clinton," he told NBC's "Meet the Press."More emails are expected to be released in coming days, and in an ominous sign for the party, DNC interim chair Donna Brazile indicated the drama was not yet over."I don't know the substance but I do know there are lots of stuff that we might have to apologize for and that's why I say you got to own it, take full responsibility and work with the staff to create a different culture at the DNC," she told CNN.Brazile said there are likely "many thousands" of leaked emails still to come.Sanders and First Lady Michelle Obama headline the first day of the Democratic convention which gavels in at 4:00 pm (2000 GMT) Monday.Bill on TuesdayFormer president Bill Clinton is the star on Tuesday, while President Barack Obama and Vice President Joe Biden take the stage Wednesday.Thousands of pro-Sanders protesters have gathered in Philadelphia, with the largest demonstration expected Monday.Many in the Sanders camp have also voiced disappointment with Clinton's choice of a centre-left running mate, Senator Tim Kaine of Virginia, instead of a more liberal firebrand like Senator Elizabeth Warren."Tim is an extremely bright guy, a very nice guy. Are his political views different than mine, yeah, they are," Sanders told CNN's State of the Union, adding that "he's a 100 times better than Trump will ever be."Clinton's campaign, meanwhile, was pushing the notion that Russia was behind the email leaks in an effort to help Trump win."Experts are telling us that Russian state actors broke into the DNC, took all these emails, and now are leaking them out through these websites," campaign manager Robby Mook told ABC. "It's troubling that some experts are now telling us that this was done by the Russians for the purpose of helping Donald Trump."