Democratic vice presidential nominee Kamala Harris warned on Sunday morning that she believes Russian election interference could pose a legitimate threat to Joe Biden’s attempt to defeat President Donald Trump. Harris, a member of the Senate Intelligence Committee that found Russia interfered to help Trump in the 2016 election, said on CNN’s State of the Union that she’s “clear” that Russia meddled before and will do it again. “I do believe there will be foreign interference in the 2020 election and Russia will be at the front of the line,” she declared.

Asked whether Russia could cost the Biden-Harris ticket a victory, Harris replied “theoretically, of course.” The California senator also said “we have to be a realist” about the possible Russian actions. It was reported last week that the Department of Homeland Security withheld a report warning of a Russian scheme to target Biden with “allegations about poor mental health.” Attorney General William Barr, meanwhile, has waved off concerns about Russia interference, saying he sees China as more of a threat to this election.