Question

What has become the most powerful predictor of how members of congress vote?

Answer 1

The most powerful predictor of congressional voting behavior is party identification. For example, if a member of congress has been elected as a Republican, there's an expectation for them to vote consistently along party lines. Although the binary in this country (Democrat / Republican), allows for little expression of actual individual political ideology, politics are a game and congress will mostly support the legislation of their "team."