The first pitched battle in the 111th Senate came to an end today when the chamber passed a compromise version of the economic stimulus bill put forth by Sens. Susan Collins, R-Maine, and Ben Nelson, D-Neb. Not surprisingly, both lawmakers have shown a tendency to hew closely to the middle since the stimulus debate began last week, as have the other two Republican senators -- Maine's Olympia Snowe and Pennsylvania's Arlen Specter -- who joined them to tip the final vote to 61-37.

Twenty-three of the 28 Senate roll call votes cast on amendments to the stimulus bill fell out largely along party lines. Most of them concerned Republican proposals to limit spending measures or introduce new tax cuts. Some, such as Arizona Republican John McCain's amendment to strike down the bill's "Buy American" provision, targeted more specific measures. Only one amendment drew a straight party-line vote -- a McCain proposal to replace the current bill with a $421 billion substitute focusing on tax cuts.

As illustrated in the chart below, seven Republican senators have crossed over to join the majority party on at least three occasions; five of these represent states that voted for Obama. In comparison, nine Democrats crossed over on three or more amendments, but they did so on average far less frequently than their GOP counterparts (7.9 times for the serial Republican swing votes and 4.3 for the Democrats).

President Obama has made a priority of reaching out to congressional Republicans, but the record of stimulus votes suggests he could have a remarkably small universe to work within. While the majority of Republican lawmakers showed a willingness to side with Democrats on at least one vote, it was usually to vote against a fellow Republican’s proposal rather than to support a Democratic measure.

Collins and Snowe shared the title of most frequent Republican swing vote, with 15 crossover votes each. They were followed by Specter, who cast his lot with the Democrats 10 times, and Ohio's George Voinovich, who did so seven times. Obama won all three states last November.

Across the aisle, independent Democrat Joe Lieberman of Connecticut and Evan Bayh of Indiana voted with the GOP most often, at seven times each. They were closely followed by Ben Nelson of Nebraska, who did so six times. These wavering Democrats will be just as crucial to hold on to in future legislative battles as the Republican swing voters.

* Denotes a Republican senator from a state carried by Barack Obama in 2008.

Table by Vincent Barranco