Sanders backs Markey in primary

Vermont Sen. Bernie Sanders is backing Massachusetts Sen. Ed Markey for reelection Monday, throwing major progressive weight behind the Bay State incumbent as he collides with Rep. Seth Moulton in a heated primary.

Why it matters: Moulton, 47, has made Markey's age — he turns 80 July 11, — and his 50-year Washington career central to his campaign.

The backing of Sanders, 84, provides Markey with a shield to help lock down the progressive base Markey has relied on in the past to keep him in office.

State of play: Markey and Sanders have been aligned in the Senate on plenty of progressive goals, like Medicare for All, workers' rights and Markey's push for the Green New Deal.

What they're saying: In a statement, Sanders called Markey "one of the strongest progressive voices in the United States Senate" who understands the struggles of working families.

Markey has consolidated support from labor, as well as Washington and Beacon Hill Democrats.

Yes, but: Moulton is catching up in the polls and out-raising the incumbent, according to Markey's own campaign.

By the numbers: Markey has served in Washington for half a century, arriving in Congress in 1976 before being elected to the Senate in 2013.

Sanders entered Congress in 1990.

What's next: Three debates between Moulton and Markey are scheduled this summer, kicking off July 8 in western Massachusetts.

The winner of the Sept. 1 primary will face Republican John Deaton in November.