Nate Silver of FiveThirtyEight advises using discretion Before You Call Your Senator, Read This On How Our Trump Scores Work. At present the sample size is small so you can expect some swings in the scores. Things should stabilize as more votes are recorded.
[Last] Monday, we launched a dashboard that tracks how often members of the House and Senate have voted in line with President Trump’s position on bills and nominations. We were pleasantly surprised at how many people1 were interested in this feature — the level of political energy right now is about as high as I can remember in the almost 10 years that I’ve covered politics. But as a result, what we’d expected to be a fairly slow rollout suddenly occurred much faster. So I wanted to add a word of caution, along with a bit more methodological detail.
The caution is simply this: For the time being, these calculations aren’t based on very many votes. Therefore, they’re likely to bounce around over the next few weeks until more votes are taken. As of Monday, they included just four votes in the House and six votes in the Senate. It’s also important to note that we aren’t tracking all votes — only those on which Trump takes a clear position.2 So they represent a small sample size, for now.
Another unique feature of our dashboard is the plus-minus scores. The basic idea is to compare how often a member of Congress voted with Trump against others where the 2016 presidential vote was similar. For instance, you’d expect members to support Trump most of the time if they come from a state or district that voted for Trump by 30 percentage points, but not very often if they’re from one where Hillary Clinton won by that margin.
For example, take our own Rep. Martha McSally (R, AZ CD2). Her district is pretty much split between Ds and Rs. So you would expect the predicted percentage of times she votes in agreement with Trump’s positions to be about 50–50. 538’s prediction is 49.2%. But her percentage in agreement with Trump is 100%. That ranks her at #13 in the House and #12 among House Republicans. So, my friends, McSally can dodge the question of whether she supports Trump or not but her voting behavior tells the story. She might be from CD2 but she is not of CD2.