With the battle over Americans for Prosperity’s misleading ad featuring a Michigan cancer victim still raging, the Dem group Senate Majority PAC is now up with a response ad backed by at least $400,000. It defends Dem Rep. Gary Peters — the Senate candidate there — and hits his GOP opponent, Terri Lynn Land, on a range of other fronts:

Dems are still getting badly outspent in Michigan. AFP has reportedly spent some $2 million in the state. The new Senate Majority PAC buy — in a state where Dems should be having a relatively easy time, at least compared to red state contexts – suggests Dems are taking AFP’s attacks very seriously.

The ad doesn’t directly engage any of the AFP ad’s claims about Obamacare, simply pointing to a series of fact checks knocking it down. Instead, it points out who is behind the ad, arguing that the Koch brothers are trying to “buy a Senate seat” for Land so she can cut taxes on the rich and “restrict women’s access to health care.” In an interview with Politico, Land said the “only exception” to abortion she favors is when the mother’s life is in danger, not mentioning rape or incest — which Dems believe is a key vulnerability.

The spot also hits Land for wanting to cut Medicare, concluding the AFP ad against her shows her devotion to “helping the powerful at our expense.”

In a broad sense, the ad is in keeping with the Dem belief — or perhaps hope — that Obamacare is not the only issue voters care about; that swing voters will perceive such attacks on the law as political and ideological; and that 2014 races will be fought out on a range of issues, despite apparent GOP certainty that Obamacare alone (AFP has repeatedly said its sole overriding goal is repeal) will be enough to deliver the Senate.

One thing that will be interesting to watch: If and when Dems seriously engage in Michigan, will it become apparent that the battle for this seat isn’t as competitive as it looks right now? If so, you’d probably see AFP stop engaging there. If the contest remains very close even after Dems engage, it will be a sign Republicans really are succeeding in broadening the map in a way that should worry Dems a good deal.

Greg Sargent writes The Plum Line blog, a reported opinion blog with a liberal slant -- what you might call “opinionated reporting” from the left.