(Phoenix, AZ) -- Greg Stanton has stepped down as mayor of Phoenix. Stanton says he believes the city is on the most positive trajectory of any other large city in America, and he's OK with resigning to prepare for a Congressional run. He's trying to fill the seat currently held by Representative Kyrsten Sinema, who is now running to fill the Senate seat currently held by Jeff Flake, who's chosen not to run for reelection. Stanton spent nine years on the Phoenix City Council before being elected to the mayor's office in 2012. Vice Mayor Thelda Williams will take over, until the city council can make a decision about an interim mayor.