The tension between the public good and re-election also highlights the dual role of members of Congress: They are national policymakers but local representatives. When they are criticized for favoring re-election over the public good, it is often a criticism of putting local concerns over national ones.

Faced with a policy that helps the national economy but costs local jobs, many members will feel obligated to defend their constituents, and oppose the policy. Congressmen routinely oppose trade deals on these grounds, and some blue-state Republicans, in the debate on the recent tax bill, faced opposition at home to altering deductions for state and local taxes.

In practice, this local incentive tends to distort public policy toward parochialism and away from the national good. But this is neither irrational nor inherently bad. Even if you believe national interests should take priority, it doesn’t follow that local interests should be ignored.

Complicating this, most citizens do not have consistent opinions about when each role should take precedence. So asking members to “do the right thing” often means asking them to risk their job for an important policy, but just this once. Even worse, determining “the right thing” is largely subjective; in most cases, it just means “a position I feel strongly about.”

The third goal of members — increasing their power within Congress — is intrinsically linked to partisanship. Power in both the House and Senate is structured along party lines, with party caucuses holding gatekeeping authority over committee seats, chairmanships and chamber leadership positions. Members can parlay skills like fund-raising prowess or issue expertise into increased power, but in most cases they need to please party leaders to improve their standing.

When all three of their incentives align, members have little to consider. If a position is strongly supported by constituents, pleases party leaders, and is judged by the member to be good public policy, then taking that position is a no-brainer.

The most difficult choices members make concern supporting the party position on issues that cut against the preferences of their constituents or district interests. This may take the form of voting with the party on high-profile legislation. For example, the Democrats in 2010 passed the Affordable Care Act even though it was obvious the votes were going to cost them seats in Congress.