NEW YORK – New Republican-leaning independent groups entered the presidential advertising fray Wednesday as polling suggests Mitt Romney’s campaign may be losing ground against President Obama in key swing states.

The commercials, aimed at voters who supported Obama in 2008 but are undecided now, join those from the campaigns and outside groups swamping a narrow map of competitive states in the presidential contest.

Americans for Job Security launched an ad in six swing states as part of an $8.7 million ad buy disclosed last week. The group, which does not have to disclose its donors, has actively supported Republican congressional candidates but has stayed out of the presidential campaign until now. The Ending Spending Action Fund, a new conservative group bankrolled by billionaire Joe Ricketts, was set to debut a $10 million, four-state ad campaign beginning Thursday.

The two groups have joined a crowded field of players in a presidential advertising landscape that has largely narrowed to nine states: Colorado, Florida, Iowa, Nevada, New Hampshire, North Carolina, Ohio, Virginia and Wisconsin. All are states Obama carried against Republican John McCain in 2008 but have been close this time.

A pro-Romney super PAC, Restore Our Future, is advertising in Michigan, while another Republican-leaning group, the American Future Fund, announced Wednesday it would run ads supporting Romney in Minnesota. But polling shows those states tilting heavily toward Obama and neither is considered a top-tier battleground this time because the candidates themselves aren’t on the air in those states.

It’s a vast change from 2008, when the Obama and McCain campaigns advertised in 21 states that were considered competitive. Several of those have seen virtually no advertising this year, including Arizona, Georgia, Indiana, Maine, Missouri, Montana, New Mexico, Pennsylvania and West Virginia. Polls show Georgia, Indiana, Missouri, Montana and West Virginia all safe for Romney this time — Obama carried Indiana in 2008 but is not expected to again because he’s not competing in earnest there — while Maine, New Mexico and Pennsylvania are expected to favor the president as they did in 2008.

The decision by Romney’s campaign and allied groups to abandon advertising in Pennsylvania particularly surprised political observers, since the state was a top battleground 2008 and is home to many of the white, working-class voters who polls show have been skeptical of Obama since his first run for the presidency. Pro-Romney independent groups including Crossroads GPS and Restore Our Future advertised there but have since pulled out, as did the pro-Obama group Priorities USA Action. Neither the Obama nor the Romney campaigns actively advertised in the state.

Elizabeth Wilner, vice president of the advertising tracking group Kantar/Campaign Media Analysis, said that except for the recent addition of Wisconsin as a battleground, the states that have seen presidential campaign advertising have not changed since the spring. Independent groups, not the Obama and Romney campaigns, have pushed to make states like Pennsylvania, Michigan and Minnesota and even Wisconsin more competitive, Wilner said.

“The cake was basically baked in May. The states you’re seeing in play have stayed the same,” Wilner said. “The candidates never made real investments in any other states. The independent groups are the first to go in to a state and the first to go out.”

The Ricketts ads were set to be coupled with radio and online advertising, direct mail and door-to-door voter contact programs led by former Bush White House political director Sara Taylor Fagen.