A victory over Flake would be a huge boost to the Democratic Party, which has few other opportunities to topple an incumbent Republican senator in 2018. A Flake loss would also be devastating for the GOP’s agenda in the Senate, where a slim majority has already hampered Republican legislative plans.

“In recent days, we’ve continued to see the Jeff Flake political stock plummet,” said Chris Herstam, a former Republican whip in the state House who has since become a Democrat.