WASHINGTON—The Republican-led House Intelligence Committee released its long-awaited report on alleged Russian interference in the 2016 election, concluding that Moscow waged a campaign to undermine the legitimacy of the U.S. democratic process, but finding no evidence that Donald Trump or his campaign assisted in the effort.

Democrats on the committee contested the Republicans’ findings, writing in their dissent that Russia sought influence in Mr. Trump’s campaign and that the campaign was open to such contact.

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