DENVER — Colorado U.S. Senate candidate John Hickenlooper has filed petitions with the secretary of state's office in hopes of qualifying for the state Democratic Party's June 30 primary. 

The former two-term governor heads a field of Democrats hoping to face Republican Cory Gardner in November. 

Gardner is considered one of the most vulnerable GOP senators seeking re-election because Colorado has become reliably Democratic in the age of President Donald Trump. 

The Democrats' Senate race also is headlined by former state House Speaker Andrew Romanoff. 

Major party candidates can petition their way onto their respective primary ballots or qualify through a process that begins with party caucuses and concludes with state assemblies on April 18. 

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