The Senate debate on the omnibus spending bill is expected to end this evening, five days and many Republican amendments later than Majority Leader Harry Reid, D-Nev., had hoped for when the process began. The past week's maneuverings have shown just how hard it will be for Democrats to corral the 60 votes needed to push forward their agenda in the Senate, and how much they'll have to rely on the fiscally conservative Democrats and moderate Republicans in the middle. Even more importantly, their task with the budget bill will be replacing the votes of defecting Democrats with enough Republican appropriators willing to vote in support of their earmarks.

As the graphic below shows, both Republicans and Democrats have demonstrated less party discipline on the budget than they did during the debate over the stimulus bill. In the 16 roll call votes so far on Republican amendments -- most of which propose to strip or restrict earmark spending, all of which fell out generally along party lines -- more lawmakers have shown a willingness to vote against their party than did during consideration of the American Recovery and Reinvestment Act in February.

Evan Bayh, D-Ind., and Ben Nelson, D-Neb., have been the least reliable votes, crossing over to vote with Republicans on 10 of the amendments so far. Both have expressed strong reservations about the legislation, although Nelson now says he will support a cloture vote. They were followed closely by Maine Republican Olympia Snowe, who has crossed over on nine amendment votes and was one of three Republicans to vote in favor of the stimulus bill. Wisconsin Democrat Russell Feingold opposed his party on eight amendment votes and plans to do so on the final vote.

On both sides, the swing votes have come from lawmakers with long histories of working from the middle. Of the 10 lawmakers who have broken with their party on a half-dozen amendments or more, nine were ranked as centrists in National Journal's vote ratings of the 110th Congress. Five of the top six hail from presidential swing states.

Not surprisingly, Republicans with seats on the Appropriations Committee were the most likely to side with the majority in favor of the legislation they helped to craft. Two GOP appropriators have joined Democrats to vote down Republican amendments eight times so far: Thad Cochran of Mississippi, the committee's ranking member, and Arlen Specter of Pennsylvania, who also voted in favor of the stimulus. Maine's Susan Collins and Missouri's Christopher (Kit) Bond have each done so seven times, and Alaska's Lisa Murkoswki has crossed over six times. In addition, appropriators Richard Shelby of Alabama and George Voinovich of Ohio have crossed over five times each.

Assuming all 58 Democrats are present for the cloture vote -- Reid brought Edward Kennedy, D-Mass., back to Washington when it became obvious the vote would be a close one -- and that Feingold and Bayh vote against it, Reid and Majority Whip Richard Durbin, D-Ill., will have to find four Republicans to pick off. Barring a concerted Republican effort to prolong the debate further, Reid and Durbin will find the votes they need among the serial swing voters: the seven GOP appropriators and Snowe.

NOTE: The chart below has been updated to reflect subsequent roll call votes and does not reflect the numbers given in the story. Seven Republican appropriators and Snowe voted on Tuesday to invoke cloture on the measure: Lamar Alexander of Tennessee, Bond, Cochran, Murkowski, Shelby, Specter and Roger Wicker of Mississippi. Their votes more than made up for the defections of Democrats Bayh, Feingold and Claire McCaskill of Missouri.