Begich was mayor from 2003 to 2009. He was elected to the Senate in 2008.
The spot, shared with Post Politics, follows the release of National Republican Senatorial Committee ads that hit Begich over his mayoral record and his senatorial resume.
Polls show a competitive race between Begich and Sullivan in a key battleground in the fight for the Senate majority.
The Republicans spots come amid an intensifying ad war that burst onto the national radar last week when Begich's campaign took down an ad holding Sullivan partly culpable for releasing a man from prison who later allegedly killed an elderly couple and sexually assaulted their grandchild. Sullivan took down a response ad on the same subject.
But Begich has not stopped hammering Sullivan's record as attorney general. He released a new TV ad over the weekend that casts Sullivan as soft on crime and criticizes his handling of a pension case. Sullivan was attorney general from 2009 to 2010.
"What kind of attorney general was Dan Sullivan?" asks the narrator of the Begich ad. "First, it was letting violent criminals off with light sentences again and again -- as little as 14 months for sexual assault. Now, Sullivan's claiming credit for saving pensions. But it was a pension fraud case where he made a bad deal with a New York financial firm."
Crossroads is spending $850,000 to air its new ad statewide. It's part of a multimillion dollar fall ad buy the group previously placed.