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It’s civics 101: Before any piece of legislation becomes law, it must first be voted on and passed by both the U.S. House and Senate. It sounds simple enough in theory, but in reality, the process is often neither simple nor straightforward.

For the last four years Larry Evans, with the help of a revolving cadre of undergraduate research collaborators, has been looking behind the generally closed doors of Congressional leadership at what he sees as one of the central investigations among political scientists today—the role that political parties and their leaders play in the legislative process. His window on this process has been the tally sheets, or “whip counts,” that congressional leaders use to track the positions of members prior to major roll call votes.

“It’s actually kind of difficult to measure the impact of leaders and parties on the legislative process,” said Evans, Newton Family Professor of Government at William and Mary. “For instance, if you are looking at a roll-call vote, if it is an important issue more often than not you are going to have a party-line outcome with the Democrats on one side and the Republicans on the other. But the question is why?”

Evans noted that political scientists can suppose numerous reasons for the frequency of the party-line split. After all, Democrats and Republicans alike display a high degree of political cohesion, both naturally ideological or driven by interest groups or constituency concerns. Evans says another possible cause is that each party’s leadership is stepping in and influencing members. The individuals who are charged with making sure there are no surprises when the votes are tallied are called “whips.” In the Senate and the House, each party has a member who serves as the whip and is assisted by many deputy whips and expert staff.

The whip’s first job is to count votes—or rather, potential votes. Before important bills are considered on the House or Senate floor, whips often see that members of their party are individually polled, in person or by e-mail or phone. The whip usually marks each member’s response in one of five categories—“yes,” “leaning yes,” “no,” “leaning no” or “undecided.” The papers logging the responses are called whip counts and are the raw material for Evans and his students.

Whips do more than just poll the members of their party. When the vote count isn’t solid, whips begin the second part of their jobs, using their influence to turn some yesses into nos—or vice versa.