Maine's Democratic primary for U.S. Senate has intensified as Gov. Janet Mills and challenger Graham Platner engage in increasingly negative campaigning ahead of their contest to face Republican Sen. Susan Collins in November.

Mills's campaign released its first attack advertisement Tuesday targeting Platner over comments he made on Reddit in 2013. The 30-second spot shows women being handed iPads displaying Platner's past online remarks that the ad characterizes as downplaying sexual assault concerns.

The primary contest pits Mills, a governor with extensive political experience, against Platner, an oyster farmer and military veteran running as an anti-establishment progressive candidate. The race has grown increasingly contentious as both candidates seek the Democratic nomination to challenge Collins, a three-term Republican senator.

Platner's campaign has responded to the attack, leading to what sources describe as sparring between the two Democratic hopefuls over the advertisement and its claims about his past statements.

The primary battle reflects broader tensions within the Democratic Party between establishment figures like Mills and outsider candidates like Platner. The winner will face Collins, who has held her Senate seat since 1997 and is considered a key target for Democrats seeking to maintain control of the chamber.