Joe Biden is endorsing longtime colleague Tom Carper as he faces his toughest primary battle in decades, a rare move by the former vice president to take sides in a primary in his home state.

Biden has agreed to record a robocall on behalf of Carper, who is seeking his fourth term as Delaware senator and has served in statewide office continuously since 1977. It will begin reaching state voters next week ahead of the Sept. 6 primary, NBC News has learned.

"If there's someone who knows me and knows Delaware, it's Joe," Carper said in a statement to NBC News. "Together we've served the First State and fought to improve the lives of countless Delawareans. I'm grateful for his friendship and his encouragement to keep standing up to Donald Trump and fighting for Delaware's families."

At first glance Biden's endorsement of Carper is unsurprising, given the decades-long partnership between the two. But Biden has rarely — if ever — taken sides in Democratic primaries in his state since being elected to the Senate in 1972.

The move also pits Biden against the ascendant progressive wing of the Democratic Party, in his own backyard. Carper faces Kerri Harris, an Air Force veteran who is seeking to marshal the same forces who helped Alexandria Ocasio-Cortez in her upset victory over New York Rep. Joseph Crowley, one of the top Democrats in the House. Harris is endorsed by Justice Democrats and the Working Family Party. The WFP has been running Facebook ads targeting Carper for his voting record on Supreme Court nominations, including a vote to confirm Chief Justice John Roberts.

“Over the last thirty-five years, Tom Carper has put the demands of big banks, corporations and pharmaceutical companies, ahead of the needs of every day working families in Delaware, far too often. He's even rubber-stamped right wing judicial nominees. We're with Kerri because she'll wake up every morning ready to fight for a Delaware that works for the many, and not deep-pocketed donors,” said Joe Dinkin, the WFP’s campaign director.

Carper has ramped up his campaigning since Crowley's defeat. He recently launched a campaign video touting his daily Amtrak trips to and from Washington, drives throughout Delaware, and endorsements from key party constituencies including the state chapters of the AFL-CIO and teachers union.

It was just eight years ago when a long-serving Republican, former governor and Rep. Mike Castle, suffered a stunning defeat to Christine "I'm Not A Witch" O'Donnell in the race to fill Biden's former Senate seat.

Biden is gearing up for a busy fall schedule of campaigning for Democrats in midterm races. He will headline the Pittsburgh Labor Day parade, a longtime favorite stop for the former VP.