Nancy Pelosi, who becomes Speaker of the House today, will govern with a 15-vote margin. Each of the muscular factions within the House Democratic family could hold her up, but they won’t because they know their majority is fragile.

Pelosi starts her historic tenure as the first female speaker with the Democrats rock-solid unified. The Blue Dogs, the New Democratic Coalition, the Congressional Black Caucus, the Progressive Caucus and the Hispanic Caucus have little chance of promoting their various — sometimes different, sometimes overlapping — agendas if Pelosi is thwarted at every turn.

Once the 110th Congress is sworn in, watch for Pelosi’s team to show-not-tell unity by supporting issues important to each alliance. Pelosi has already given the budget-hawk Blue Dogs their due by embracing one of their central issues. The new Rules package includes a pay-as-you-go requirement, nicknamed PayGo. The Blue Dogs’ PayGo provision will be showcased on Friday.

Pelosi is pragmatic. There is a very real need to make a record that Democrats can run on in 2008 to hold their power and add seats.

“I think there is an overwhelming sense for all Democrats that they are going to succeed or fail together,” Simon Rosenberg told me. Rosenberg, the president of the centrist New Democrat Network, was involved in the establishment of the New Democratic Coalition chaired by Rep. Ellen Tauscher (D-Calif.).

“There is no option of succeeding without Nancy Pelosi succeeding,” Rosenberg said.

Pelosi has been explicit in saying in various interviews that in her new role she is the leader of the entire House; in her own Democratic tent she is the ringmaster. The groups splinter at their peril. Those who portray Pelosi as a San Francisco lefty don’t get that as leader, Pelosi takes herself to another level.

A testimony to the harmony is that the progressive wing, which counts Pelosi as one of its own, is notably at peace with the PayGo requirement. “As a whole, groups like ours want expanded public investment,” Roger Hickey told me. Hickey, looking at the bigger picture, has no problem giving the Blue Dogs their PayGo due. Hickey is the co-director of the progressive Campaign for America’s Future, which is leading a coalition of outside groups working to pass much of the House-of-Pelosi legislative agenda.

As a practical matter, much of the debate will end up being on discretionary spending and the larger items may have to wait for another day no matter what gets passed out of the House. That’s because the Democrats have a tenuous Senate grip and it basically takes a supermajority to get anything done. Democrats in Pelosi’s House, therefore, will not bother having messy public fights on symbolic legislation that may well end up being stalled in the Senate.

“The omens are good for unity among the Democrats, particularly because they feel they have been given a mandate on the war in Iraq and the domestic economy,” Hickey said.

Ellen Malcolm, the president of Emily’s List, predicted the Democrats will remain unified. Said Malcolm, “They got to get stuff done and they know it.”

Sweet is the Washington bureau chief for the Chicago Sun-Times. E-mail: This email address is being protected from spam bots, you need Javascript enabled to view it