President Obama’s campaign is up with a new ad contrasting his tax plan with Mitt Romney’s and accusing Romney of wanting to raise taxes on 18 million Americans.

The ad accuses Romney of supporting tax breaks for the wealthy, for oil companies and for those who ship jobs overseas, along with a tax increase for 18 million “working families.”

The ad represents the Obama campaign’s decision to double down on its message of raising taxes on the wealthy by letting the Bush tax cuts expire. But it also shows a desire to open up a new front by attacking Romney as a potential tax-raiser too.

The tax increase for 18 million families is a claim that Obama has made before, and the fact-checking Web site PolitiFact has rated it “mostly true.” The tax increases come as a result of allowing tax cuts from the stimulus package to expire. (Read more here.)

The ad is airing in Colorado, Florida, Iowa, Nevada, New Hampshire, North Carolina, Ohio, Pennsylvania and Virginia.

Update: Here’s the Romney campaign’s response, from spokeswoman Amanda Henneberg: “Over the past four years, it’s become clear that President Obama doesn’t have a clue how to get America working again or help the middle class. President Obama’s tax increase will hit families, job creators, and small businesses all over the country. Governor Romney understands that the last thing we need to do in this weak economy is raise taxes on anyone. He has a plan to permanently lower marginal rates, help middle-class Americans save and invest, and jumpstart economic growth and job creation.”