WASHINGTON — Mitt Romney's policy shifts moved to the center of the presidential campaign Monday as the former Massachusetts governor fought back aggressively after Democrats' mockery of his changed positions on a dozen issues including abortion, guns and immigration.

Even with GOP rival Newt Gingrich enjoying a spate of good news, Romney and President Barack Obama appeared more focused on each other than ever, as if the general election already were under way.

Romney's campaign quickly arranged conference calls in several states to combat a new Democratic TV and Internet ad that depicts him as a politician lacking core values. Republicans said Obama just wants to turn attention away from the weak economy, but the urgency of the Romney reaction suggested his campaign sees the flip-flop accusations as serious.

Details of Romney's shifts on key issues are not new. Yet they have played only a peripheral role in the eight-person GOP nominating contest so far, to the dismay of some Democrats.

In a career that includes an unsuccessful Senate race and one term as governor in Massachusetts, plus a 2008 presidential bid, Romney at times has favored legalized abortion, a ban on assault weapons and a pathway to legal status for some illegal immigrants.

He since has rejected those views. He also takes a harder line than before on government stimulus programs and bank bailouts. Romney's health care initiative in Massachusetts required residents to obtain medical insurance, but he rejects the notion that it was a model for Obama's national plan enacted last year.

Over the years, Romney has minimized the significance of some of his shifts. He attributes others to heart-felt changes of opinion.

The Democratic National Committee on Monday launched a multi-state attack, portraying Romney as a politician in conflict with himself. A TV ad is airing in the battleground states of North Carolina, Virginia, Pennsylvania, Ohio, Wisconsin and New Mexico. Democrats also held events in Iowa, Florida, Michigan, Maryland and Massachusetts to call attention to a longer and more detailed version of the criticisms on the website mittvmitt.com.

The video calls Romney "an unparalleled flip-flopper." It shows two late-night comedians mocking his sincerity and three Fox News reporters seeming to question Romney's authenticity.

Romney's campaign responded with conference calls featuring current or former Republican officials from Ohio, Wisconsin, New Hampshire, Pennsylvania, Iowa, North Carolina and Florida. In general, their retorts focused on Obama's economic record rather than on point-by-point efforts to defend Romney against flip-flop charges.

Tim Pawlenty, a former Minnesota governor who endorsed Romney after dropping his own presidential bid this year, said Obama has failed to create jobs or reduce the federal debt. "This administration does not want to campaign against Mitt Romney and be forced to defend three years of failure," Pawlenty said.

In one of the conference calls, Rep. Patrick McHenry, R-N.C., said Romney "has stated clearly that his position has evolved" on abortion. McHenry said he is satisfied that Romney would be a staunchly anti-abortion-rights president.

Democratic activists said it's unlikely their ads will significantly influence the GOP primary, even though they would prefer to run against someone other than Romney. They said Obama can't afford to wait to begin undermining Romney among independent voters in battleground states.

Their strategy assumes Romney will emerge as the nominee, even though polls show him struggling to lock down the support of more than one-quarter of Republicans. The Iowa caucuses will be held Jan. 3, with the New Hampshire primary coming a week later. January contests in Florida and Nevada will follow.

Gingrich, a former House speaker, won the sought-after endorsement Sunday of the Union Leader, New Hampshire's largest newspaper and a prominent conservative voice in the state. Gingrich, who represented Georgia in Congress for two decades, is campaigning this week in South Carolina.

He hopes to avoid the type of momentum losses suffered earlier this year by Rep. Michele Bachmann, R-Minn., Texas Gov. Rick Perry and businessman Herman Cain after they rose to the top of Republican polls alongside Romney.

Perry is to campaign in New Hampshire this week, while Bachmann is in South Carolina.

Monday's exchanges between allies of Obama and Romney marked the second case of sharpened rhetoric in recent days. Last week, Romney's campaign acknowledged that an Obama quote used in a TV ad was taken out of context but called the tactic fair. Democrats bitterly criticized the ad.

Romney generally answers accusations of flip-flopping by diminishing his shifts in views or calling them old news.

The new DNC ad shows an undated video clip of him addressing abortion and saying, "I will preserve and protect a woman's right to choose."

In early 2007, Romney said he changed his view on abortion after meeting with a stem cell researcher.

"The comment was made that this really wasn't a moral issue because the embryos were terminated or destroyed at 14 days," Romney said at the time. "And it struck me very powerfully at that point, that the Roe v. Wade approach has so cheapened the value of human life that someone could think it's not a moral issue to destroy embryos that have been created solely for the purpose of research." Romney said he told an aide, "I want to make it very clear that I'm pro-life."

Roe v. Wade is the landmark 1973 court decision that barred states from outlawing abortion in most instances.