Groups pushing health care reform are readying a multimillion-dollar ad blitz to defend House Democrats who helped pass a sweeping overhaul and now are being targeted with attack ads.

The largely labor-funded counterpunch comes after anti-reform groups surprised Democrats with a similarly well-funded and highly concentrated strike right after the House vote on Nov. 7. Democratic allies have already shelled out for a series of thank-you ads, but they found themselves spread comparatively thin across about 30 districts while reform opponents pounded away in about a third as many districts.

Details of the new pro-reform campaign were still being ironed out Tuesday, but it will include a weeklong $750,000 television ad buy by the American Federation of State, County and Municipal Employees and Americans United for Change targeting 12 House Democrats; a $1 million offensive by the Service Employees International Union aimed at a partially overlapping list of seven Democrats; and a separate effort by AARP that the organization declined to detail.

Among the House Democrats getting help: Baron P. Hill , Brad Ellsworth and Joe Donnelly of Indiana; Marion Berry and Vic Snyder of Arkansas; Gerarld E. Connolly and Tom Perriello of Virginia; Christopher S. Murphy of Connecticut; Paul W. Hodes of New Hampshire, Michael H. Michaud of Maine; Earl Pomeroy of North Dakota; and Dina Titus of Nevada.

CQ Politics rates the seats held by Pomeroy, Michaud, Berry, Ellsworth and Donnelly as “Safe Democratic” at this stage in the 2010 race; the seats held by Murphy, Snyder and Connolly as “Likely Democratic,” and the seat held by Titus as “Leans Democratic,” a highly competitive rating. Perriello’s seat is rated a “Tossup.” Hodes is running for the Senate.

Also on the list is the lone Republican to vote for reform, Rep. Anh “Joseph” Cao of Lousiana. CQ Politics views his seat as likely to turn over; it is rated Likely Democratic

All the lawmakers are from states with senators who are considered potential swing votes when that chamber takes up its version of the overhaul — a fact opponents and supporters alike say is no accident.

The push to provide late air cover comes after a bruising at the hands of the U.S. Chamber of Commerce, which has dumped a multimillion-dollar sum into ads, and the 60-Plus Association, a conservative seniors’ group.

“We were surprised by the scale,” said Richard Kirsch, national campaign manager for Health Care for America Now, an umbrella group that helped underwrite the first $1 million campaign of thank-you ads in the days following the vote. “This is a new phenomenon here, the intensity of it.”

House Democratic leaders learned that lesson the hard way over the summer when they were caught flat-footed after the climate change vote. Industry groups opposed to the bill excoriated vulnerable Democrats who lined up behind the measure, and unprepared environmental groups mounted no immediate counter-assault to provide them cover. As a result, Democratic leaders thought they would avoid a similar outcome in the fight following the health care bill’s passage by putting outside groups on notice beforehand.

Congressional staffers said reform supporters are nevertheless having trouble marshaling the resources of their opponents. “I wish I could criticize the groups for not doing anything,” a Democratic aide said, “but I think they are and just don’t have the money.”

The opposition push was threatening enough that at least one lawmaker caught in its cross hairs — Pomeroy — decided to take matters into his own hands: He tapped his campaign funds to cut an ad defending himself, fully a year before the 2010 elections. The Pomeroy spot points to a review by FactCheck.org that disputes a 60-Plus ad accusing him of betraying seniors by supporting the health care bill.

“We’re transitioning from what has been an ad campaign about policy differences to one taking on much stronger political characteristics,” said Evan Tracey, president of Campaign Media Analysis Group, which tracks spending on political spots. “These ads are going to be an afterthought in November 2010, but they send a signal that this is what you can expect, because in most of these districts, the race is going to be a referendum on this vote.”