Turkish authorities must provide sufficient evidence of corruption for which jailed Istanbul mayor Ekrem Imamoglu is accused, or release him immediately, U.S. Senator Adam Schiff said today, urging members of Congress to support a resolution against President Tayyip Erdogan for what Schiff called a democratic setback in Turkey.

Erdogan’s main rival, Mayor Ekrem Imamoglu, was jailed in March pending trial on corruption charges, sparking Turkey’s biggest protests in a decade. Popular support for Imamoglu has since grown, with his ratings exceeding Erdogan’s, according to polls.

The mayor denies all charges. His arrest has caused turmoil in markets and accusations of a politicized judiciary, charges the government rejects, saying judges are independent.

The Sif resolution, which was obtained by Reuters, calls on Erdogan and law enforcement authorities to release Imamoglu or present credible evidence against him and uphold democratic values.

“The continued detention of Mayor Imamoglu, as well as the harassment and arrest of dozens of journalists and dissenting political voices in Turkey, is the latest in an unfortunate trend of democratic regression that Turkey is experiencing under President Erdogan,” Schiff told Reuters.

“The Senate should send a strong and clear message that the unjustified imprisonment of Erdogan’s political opponents is not acceptable.”

The resolution also calls on U.S. Secretary of State Marco Rubio to speak openly about the detention and confront Turkey over what he called undemocratic behavior.

The State Department has said little about the Imamoglu case, calling it an internal judicial matter for Turkey and calling for respect for human rights.