"The race for DNC Chairman was, of course, totally 'rigged'," Trump wrote on Twitter, one day after a gathering of Democratic party faithful elected Thomas Perez -- Obama's former labor secretary -- to head the Democratic National Committee.
Trump suggested that party insiders had fixed the vote for Perez, bending to the wishes of Hillary Clinton, his vanquished opponent in last year's presidential election.
The runner-up in the chairman race, US Congressman Keith Ellison, had been supported by Senator Bernie Sanders, who was defeated by Clinton in last year's fight for the Democratic Presidential nomination.
"Bernie's guy, like Bernie himself, never had a chance. Clinton demanded Perez!" Trump wrote on Twitter.
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Democrats chose Perez, 55, an establishment figure, over the more left-leaning Ellison to lead the fight against Trump and the Republican Congress.
A Trump tweet sent Saturday shortly after the vote read: "Congratulations to Thomas Perez, who has just been named Chairman of the DNC. I could not be happier for him, or for the Republican Party!"
Perez, the party's first Hispanic-American leader, named Ellison to the post of deputy chairman. Ellison, an African-American, was Congress's first Muslim lawmaker.
Perez fired back at Trump's accusations about the legitimacy of his election, and said he and Ellison will work closely together.
"We are united, and our unity is our greatest strength and frankly, our unity is Donald Trump's greatest nightmare," Perez told NBC's Meet the Press program.
The close-fought leadership fight was seen by some as a proxy battle between the supporters of Clinton and Sanders, as Democrats struggle to regain their footing after losing the White House and failing to reclaim the House and Senate in November's election.
