Augusta, Maine - Maine Senate candidate Graham Platner has received the endorsement of New Mexico Senator Martin Heinrich, who praised his focus on "delivering for Mainers, not billionaire donors."

"He's building a movement around folks who work hard for their family and community - folks who deserve a senator fighting in their corner," Heinrich wrote in an impassioned endorsement of Platner on X. "I'm proud to endorse and help send him to the Senate in November."

He is the third senator to back Platner and defy Senate Minority Leader Chuck Schumer, who has endorsed the oyster farmer's primary opponent, Maine Governor Janet Mills.

Platner has held the endorsement of Vermont Senator Bernie Sanders since he launched his campaign in August last year. He is supported by numerous labor unions and other progressive groups.

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On Monday last week, he also received the endorsement of Arizona Democratic Senator Ruben Gallego, who called him "the kind of fighter Maine hasn't seen in a long time."

Current polling puts Platner well ahead of Mills and gives him a significant lead over his would-be opponent in a general election, incumbent GOP Senator Susan Collins.

Throughout the campaign, the 40-year-old has criticized the for-profit health care system and has slammed President Donald Trump's climate change policy. In recent weeks, he has voiced strong opposition to the war with Iran.

"Honored to receive another endorsement from a future colleague," Platner wrote on X in response to Heinrich's endorsement. "I look forward to working with [Heinrich] to end forever wars and pass Medicare for All."