Las Vegas - Navigating ongoing rifts on the political left, Democratic Party chair Tom Perez said party unity is crucial in the fight against US President Donald Trump, whom he lambasted as an "existential threat" to the nation."We have the most dangerous president in American history and one of the most reactionary Congresses in American history," Perez said as he addressed the first national party gathering since his election in February.The former Obama cabinet officer blistered "a culture of corruption" that he said extends to Trump's cabinet, House Speaker Paul Ryan and Senate Majority Leader Mitch McConnell, but he warned that internal ruckuses over party priorities and leadership would distract from the goal of winning more elections to upend Republicans' domination in Washington.The chair's plea comes amid a rift over his appointments to little-known but influential party committees and the 75 at-large members of the national party committee.Perez and his aides plug his choices as a way to make the DNC younger and more diverse, but the moves also mean demotions for several prominent Democrats who backed Bernie Sanders over Hillary Clinton in the 2016 presidential primaries and then supported Minnesota Representative Keith Ellison over Perez in the postelection race for party chair.Routine party politicsPerez spent considerable time during the week's proceedings meeting privately with some frustrated DNC members, including some he did not reappoint.He apologised publicly on Saturday for not reaching all of those members before he announced his appointments, but he defended his overall aim."If someone ever asks you which wing of the party you belong to, tell 'em you belong to the accomplishment wing of the Democratic Party," he said."Because you're trying to get shit done. That's what we're trying to do here, folks. We're trying to move the ball forward."To some extent, the latest developments reflect routine party politics after an unusually contentious chairperson's race, but they also highlight lingering resentments from Sanders backers who accused the DNC in 2016 of stacking the nominating process in Clinton's favour.Perez's appointees will hold sway over setting the primary calendar in 2020 and, perhaps most importantly, whether the party's superdelegates, including the 75 at-large members, will continue to cast presidential nominating votes at Democratic conventions without being bound to any state primary or caucus results.Democrats are looking in November to hold the Virginia governor's seat and wrest the New Jersey governor's seat from Republican control.In 2018, Democrats need to flip at least 24 Republican congressional seats to regain control of the House.They face an uphill battle in gaining control of the Senate, because they must defend 10 incumbents in states Trump won last November. Democrats also want to increase their gubernatorial roster from the current 15 state executives.