Speaker Nancy Pelosi (D-Calif.) needs to watch the right flank of her Caucus.

United by the common threat of liberal strong-arming, the moderate Blue Dog and New Democrat coalitions are proving they are serious about mounting a sustained, coordinated pushback.

The groups are counting on the size of their ranks — a combined super bloc of 101 Members — to ensure Pelosi doesn’t use her expanded Democratic majority to swerve left.

The latest example came last week, when the moderates, led by New Democrat Chairwoman Ellen Tauscher (Calif.), compelled House leaders to punt on a mortgage bill with a bankruptcy provision that troubled the business-friendly lawmakers.

“As everyone knows, there’s strength in numbers,” said Rep. Ron Kind (Wis.), a New Democrat leader, echoing what has become a common refrain for Members of the two groups.

The moderates’ objections to the mortgage measure last week forced Democratic leaders to make changes and, later, to postpone floor consideration to allow for more talks about details of the package.

While New Democrats got credit for a win on the mortgage bill, sources close to the group said they got support behind the scenes from leading Blue Dogs. On Thursday, 13 Blue Dogs, three New Democrats and five lawmakers who belong to both groups voted against the rule governing debate on the package. That was a rare rebuke of leadership on a vote that is typically considered a test of party loyalty, especially considering that with negotiations on the package ongoing, the vote was effectively moot.

The combined effort marks an early warning shot across Pelosi’s bow: Moderates are looking to tag-team coming debates over financial regulatory reform, health care and global warming, among others, and, where necessary, muscle the discussions back toward the political center.

“I promise you, there will be lots of stuff where we’re working together,” said Rep. Allen Boyd (D-Fla.), a leading Blue Dog.

The alliance is still largely unproven. Both groups have at times struggled to maintain unity in their own ranks. They remain separated by substantial cultural and stylistic differences, and their legislative interests only intersect at select points.

But a shared fear of a leftward lurch in the House has helped re-energize what for years have been faltering attempts at closer cooperation. One significant development: Staff in both factions have begun discussing working together on efforts to protect their endangered incumbents. Sources close to those efforts said aides to Majority Leader Steny Hoyer (D-Md.), a moderate champion, have been facilitating the push.

During the last Congress, moderates relied on Rep. Rahm Emanuel (Ill.), himself a New Democrat, to quarterback the effort. The sharp-elbowed Chicagoan’s exit to become President Barack Obama’s chief of staff left Rep. Chris Van Hollen (D-Md.) tasked with looking out for vulnerable lawmakers. But sources in both moderate camps said they do not yet have faith that Van Hollen, who hails from a liberal district encompassing Washington, D.C., suburbs, understands their profile.

“There’s a recognition that we’re going to have to do more on our own,” said one aide to a New Democrat.

The new spirit of cooperation sprung up shortly after the elections placed Democrats in firm control of all levers of power.

Back in November, with a newly padded House majority, Pelosi allies were moving quickly to assert themselves: Rep. Henry Waxman (D-Calif.) announced his surprise challenge for Energy and Commerce Chairman John Dingell’s (D-Mich.) gavel on the day after the election, and Reps. John Larson (D-Conn.) and Xavier Becerra (D-Calif.) emerged early as odds-on favorites for leadership posts that opened after Emanuel quit the chamber to work for Obama.

Members of both moderate groups recognized the developments as twin threats to their influence and soon joined forces to push back. They coordinated an effort to rally support for Dingell, and senior Blue Dogs made calls to Rep. Joe Crowley (N.Y.) — a New Democrat leader well-liked by a broad swath of the caucus — lobbying him to jump into the race for Caucus vice chairman.

Both efforts fell short, as Waxman ousted Dingell and Crowley opted against a leadership bid. But moderate Members and staff from the factions kept up the cross talk.

It bore fruit early this year, after Pelosi outraged members of both groups by fast-tracking an economic stimulus package in a process they felt left them little input and then forced them to vote for a measure stuffed with politically embarrassing pork. Aides to leaders in both groups worked together on drafting a letter to Hoyer, urging leaders to loosen the reins on the legislative process. They sent it on the eve of the House Democrats’ annual retreat, and Pelosi made clear she had heard them loud and clear, welcoming lawmakers to the conference by promising a return to regular order.

So far, coordination between the factions has been strictly informal, occurring in casual conversations between Members on the House floor and phone calls between top staffers.

Kind suggested it could soon go further. “There may be opportunities where we join the groups for joint meetings and joint briefings. I could foresee certain opportunities for that popping up,” he said.

The groups remain distinct. Blue Dogs hail from socially conservative, rural and largely Southern districts. New Democrats tend to represent socially progressive, wealthier, suburban areas.

On policy, whereas Blue Dogs are laser-focused on their mission of defending pay-as-you-go budgeting, New Democrats are developing a wide-ranging and detail-laden legislative blueprint to promote free trade, encourage high tech and push financial regulatory reform that aims to add stricter oversight of the markets without stifling competitiveness.

Stylistically, Blue Dogs have set themselves up as antagonists to leadership who flex their power with their votes, while New Democrats are more likely to work behind the scenes to reach accommodations.

But perhaps more importantly, they share political DNA. Tauscher was a Blue Dog before leading a resurgence of the New Democrats in 2005. Similarly, Adam Pase — the executive director of Tauscher’s group, who is widely credited with bringing a sharp organizational focus that has reinvigorated the group — came to the coalition last year from the Blue Dogs, where he served as a top policy adviser. Fifteen lawmakers claim membership in both groups.

“As with a lot of things in politics, this is based on relationships and trust and informality and conversations,” Kind said.