In response to Friday's post on the political dynamic shaping up in the House over the economic recovery plan, I thought it would help to take a similar look at the Senate. What are the Democratic fault lines that will help determine the cost and the contents of the stimulus bill as the push-and-pull over the package continues?

The Leaders: Thanks to their plum perches, these senators have the most ability to shape the final stimulus bill, and their perspective on the right approach to the plan -- however parochial or short-sighted it may seem from outside the Capitol -- is likely to have the biggest influence on an Obama team that is eager to make nice with Congress this week. The catch with this group is that their buy-in on the bill is arguably as large as the president-elect's, so any element of the recovery plan that arouses the ire of enough rank-and-file Dems has a shot at getting nixed by one of the leaders. One thing you won't find these senators doing is publicly detailing many of the stimulus projects they prefer; but rest assured, they'll get what they want from the bill. Members include Senate Majority Leader Harry Reid (D-NV), Senate Majority Whip Dick Durbin (D-IL), Senate Democratic Vice Chairman Charles Schumer (D-NY), Senate Democratic Secretary Patty Murray (D-WA), Senate Finance Committee chairman Max Baucus (D-MT)

The Gang Members: These senators don't have matching jackets or secret signals, but they have all been members of past bipartisan "gangs", informal gatherings to seek middle ground on contentious issues such as judicial nominees and energy. In short, these are the centrists -- the Democrats most likely to break the party base's heart by tacking rightward during negotiations on the stimulus bill. Not all compromise-building efforts are counter-productive, however; the most visible member of this faction, Sen. Ben Nelson (D-NE), is getting Republicans on board to push for health care infrastructure spending as part of the stimulus. Members include Nelson, Blanche Lincoln (D-AR), Joe Lieberman (D-CT), Mark Pryor (D-AR), Mary Landrieu (D-LA), Tom Carper (D-DE).

The Senior Progressives: The Senate's true-blue core can be counted on to push for a maximum emphasis on job-creation programs and long-term infrastructure spending in the stimulus bill. These Democrats shrug at the reported $300 billion in tax cuts included in Obama's proposal to help win Republican support -- they point out that giving aid directly to states and unemployed Americans is just more efficient than doling out tax breaks. But these senators have been around for long enough, and are excited enough to see Obama take office, that they'll likely put as much pressure on the incoming administration as possible before taking what they can get from the stimulus. Members include Tom Harkin (D-IA), John Kerry (D-MA), Byron Dorgan (D-ND), Patrick Leahy (D-VT), Chris Dodd (D-CT), Jeff Bingaman (D-NM)

The Populist Progressives: These are the wild cards, the senators who have shown a willingness to buck their leadership and the K Street line when a policy strikes them as just plain unfair to their constituents. Some of them opposed the Bush administration's $700 billion bailout; others, like Missouri's Claire McCaskill, are oversight addicts who will chafe at any bill they consider too opaque. At this point, no one in Washington believes the stimulus plan will be defeated -- but if it is, these Dems will be aligning with Republicans to bring it down. Members include McCaskill, Russ Feingold (D-WI), Jon Tester (D-MT), Jim Webb (D-VA) Ron Wyden (D-OR), Bernie Sanders (I-VT), incoming Sen. Kay Hagan (D-NC)