Real Clear Politics (RCP) and FiveThirtyEight (538) are the two most relevant election analysis publications. They are dedicated to studying and analyzing the latest polls, which allows them to come as close to a consensus as we have on each elections' polling data.
Despite using similar sources and analytical tools, their conclusions are not always the same. In fact, they disagree on the principle question of the 2022 midterm elections: which party will control the Senate?
RCP's projection is that the Republicans will regain control of the Senate, winning 52 seats to the Democrats' 48. 538 believes that the Democrats will win a slim 51-49 majority. The difference is important from a numerical point of view, but politically it is abysmal. Who is more successful will not be known until November 8. But why do these differences occur?
Survey bias
Election Central explored why the two poll analysis sites' conclusions differ: "While the polling average doesn’t necessarily support Republican Senate victories in either Arizona or Pennsylvania, RCP says that inherent bias which underestimates GOP candidates in both states could produce a victory in November."
In Pennsylvania, RCP projects an average lead of 3.4 points for Democrat John Fetterman. However, according to the site's calculations, there is a 5.9-point polling bias that favors the Democratic Party, so it predicts a victory for Republican Mehmet Oz. RCP calculates a 2.2-point polling bias against Republicans in Arizona, so that race is in reality more open than it might seem.
Nate Silver, editor of 538, does not believe that survey biases are as significant, so his website does not make corrections. Election Central's evaluation is as follows: "If polls are to be taken as mostly 'accurate' without any of the massive undercounting of Republicans witnessed in recent cycles, including last year by double-digits in New Jersey, then the 538 model seems reasonable given the latest Senate polling data."
Of the 10 states where the Senate majority is at stake, there are four hotly contested races (Pennsylvania, Arizona, Nevada and Georgia) that can tip the scales one way or the other. Hot-button issues like the economy and inflation do not favor Democratic candidates, but anything can happen on election day.
