Schatz was picked to replace Sen. Daniel Inouye (D-Hawaii) after the long-time senator's death late last year, despite Inouye's deathbed request that Hanabusa replace him. She decided earlier this year to run anyway, creating a potentially contentious primary battle in the state. So far, Schatz has had no problem outpacing her in the battle for cash.

The winner of the primary in the heavily Democratic state will likely have a strong chance at holding the seat for years afterward — Hawaii has traditionally reelected its federal office-holders for decades on end.