We've got two of the most competitive congressional races in the country in Southern Arizona: Congressman Ron Barber is likely to face a rematch against Republican Martha McSally, who lost to Barber by just a few thousand votes in 2012, and Congresswoman Ann Kirkpatrick will face whoever comes out of a three-way GOP primary that includes House Speaker Andy Tobin, state Rep. Adam Kwasman and rancher Gary Kiehne. (Kirkpatrick's district is largely in eastern and northern rural Arizona, but it does include parts of Marana and Oro Valley.)

It's only January, but the battle in these districts is already underway. Americans for Prosperity has spent about $650,000 on ads that target Barber and Kirkpatrick on the topic of the Affordable Care Act.

Now the House Majority PAC, which supports Democratic candidates, is entering the fray. The group will spend about $200,000 running ads in Phoenix and Tucson for the next week.

“The billionaire Koch brothers have been spending freely to promote an agenda that is decidedly anti-family, anti-worker and anti-senior,” House Majority PAC spokesman Andy Stone said in a press release. “House Majority PAC is standing up to make sure those misleading and deceptive attacks do not go unanswered.”

Here's the ad that will run in Phoenix in defense of Kirkpatrick:

While the Americans for Prosperity ads are similar in their focus on the Affordable Care Act, the House Majority PAC ads take different tacks, perhaps because the group doesn't yet know who Kirkpatrick will face, but it's more than likely that McSally will be the candidate down here, so the ads can both target McSally's previous support for privatization of Social Security and blast the Koch brothers, who are funding the AFP ads. Meanwhile, the Kirkpatrick ads focus on her criticism of the rollout of the Affordable Care Act's website, as well as her support for provisions in the ACA that prevent insurance companies from discriminating against people with pre-existing conditions.

While these ads represent the start of the battle for these congressional districts, it's unclear as to whether they do much in terms of affecting the outcome of the elections. Stu Rothenberg recently expressed skepticism about the impact of early advertising in a Roll Call column:

But while early TV messages may get through, there is no reason to believe that they will have much staying power. Voters’ attention spans are short, so unless those early ads stay on the air continuously through Election Day, or an individual spot is unusually creative and memorable, the messages in those ads are likely to fade quickly.