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

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."

The truth is that any source can be the best source for some kind of research. as an example, if you were to write an essay about the credibility of wikipedia, would you avoid citing wikipedia? clearly not. as we know, the president of the united states jumps on twitter all the time. if you’re writing about trump, what better source can there be? it is, quite literally, an original historical document. the same would be true if you were writing about the events of the moderately recent revolutions in egypt and syria. what better source than someone who is right there in the thick of things? clearly you would have to contextualize the source as at least moderately unverified in most such cases.