Case has long had a testy relationship with establishment Democrats both in Hawaii and in D.C. He ran against Akaka in a 2006 primary, earning his enmity and that of the state's kingmaker, Sen. Daniel Inouye (D-Hawaii). Inouye has attended at least one Hirono campaign event this year, and while he said he won't formally endorse in the primary, he said he is "going to vote for" her.

On Tuesday, Murray followed suit.

"I'm supporting Mazie, and I believe she is going to win," she told reporters at a press briefing. That followed a rare public rebuke from the DSCC of one of its candidates last summer, when it slammed a poll Case put out that showed him as being more capable of winning the general election.

The winner of the primary will face former Hawaii Gov. Linda Lingle (R), who could be a formidable opponent in the race.