Democratic Gov. and newly-minted Senate candidate Maggie Hassan

Democratic Gov. and newly-minted Senate candidate Maggie Hassan

The long-awaited duel between New Hampshire Republican Sen. Kelly Ayotte and Democratic Gov. Maggie Hassan is on! On Monday, Hassan announced that she would challenge Ayotte in a contest that could very well determine control of the U.S. Senate. Independent polls have usually shown a tight race with perhaps a small edge for the GOP.

Both parties agree that Hassan is the strongest possible Democratic challenger for Ayotte. Hassan has usually posted high favorable ratings, and she won re-election 52-47 even as the GOP wave was hitting New Hampshire last year. Ayotte's allies spent millions airing ads against Hassan, hoping that they could deter her from running (or at least drive down her favorable ratings). Democrats also didn't have a compelling fallback contender if Hassan said no.

New Hampshire is a very competitive state and it has plenty of voters who are willing to split their tickets, and neither Ayotte nor Hassan can take anything for granted. Ayotte hasn't been particularly popular in recent years but she is a formidable candidate, and she won't go down without a fight. Ayotte is also a strong fundraiser: She raised $1.6 million over the last three months and has a hefty $5 million on hand, and Hassan will need to play catch up. But Hassan gives Team Blue the chance to score a critical pickup, and both sides are going to spend whatever they need to to win here.