State Representative Carl M. Sciortino, who has worked vigorously to position himself to the left of his six Democratic opponents in the race to succeed Edward J. Markey in Congress, today announced a double-endorsement that should burnish his liberal credentials.

The co-chairman of the Congressional Progressive Caucus, the umbrella group for left-leaning Democratic members of Congress, both said they were backing the Medford Democrat in his race for the Fifth Congressional District.

US Representatives Raúl M. Grijalva and Keith Ellison, Democrats of Arizona and Minnesota, respectively, released a joint statement lauding Sciortino, released by his campaign.

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“Carl Sciortino is the real thing,” the congressmen said. “He is the consistent progressive in the race, having led the fight for the issues we progressives care about for almost 10 years in the state legislature...”

A Grijalva aide confirmed the endorsement.

The other Democrats running for their party’s nod in the heavily Democratic Fifth District are: state Senators Karen Spilka of Ashland, Will Brownsberger of Belmont, and Katherine Clark of Melrose; Middlesex Sheriff Peter J. Koutoujian of Waltham; Martin Long, an Arlington author; and Stoneham resident Paul John Maisano, who works in the construction industry.

There are three Republicans running for their party’s nomination: actuary Tom Tierney of Framingham, Harvard nanophysics researcher Mike Stopa of Holliston, and businessman and lawyer Frank J. Addivinola Jr. of Boston.

The congressional seat became vacant after Markey was elected to the US Senate in June.