Followers of Leo, The Upshot's Senate model, will note that it has a new background color today.
When we created Leo, we decided, somewhat arbitrarily, that 65 percent would signify the crossing from a "slight edge" to a "moderate edge." Our latest forecast gives the Republicans a 67 percent chance of winning the Senate, up from 64 percent on Monday.
One thing we've hoped to impart with our work is that 67 percent is not meaningfully different from 64 percent. In a sense, the crossing of this threshold signifies very little beyond the fact that our poster has changed from yellow to pink. At the same time, our forecast is also at its most Republican-leaning point yet this cycle. So perhaps it is worth exploring why.
Recent polls are one big reason. As we discussed last week, the latest evidence from Georgia has been favorable for Republicans. In West Virginia, polling continues to suggest that Natalie Tennant, the Democratic secretary of state, is a long shot to win. In Alaska — an important state for Democrats to hold — a Rasmussen poll released on Monday suggested the race was close. (Polls from Rasmussen were Republican-leaning in 2012, but these house effects are not very consistent between cycles. This cycle, we estimate Rasmussen polls have been perhaps a little more Republican-leaning than those of the typical pollster, but not by much.)
Another reason for the Republican gains in our model is the mere passage of time. Every day that passes with things remaining roughly as they are will gradually increase the leading candidate's forecasted probability of winning. This is simply because there are incrementally fewer opportunities for a campaign-changing gaffe, or some other event that could swing the polls.
It's analogous to a football game in which one team is up by 3 points with the ball at its 40-yard line. Given this situation at the start of the second half, we can forecast, based on historical outcomes, that the leading team has a 67 percent chance of victory. But that 67 percent morphs into an almost certain victory with only one minute left on the clock.
Thus, with every passing day, in the absence of new information, we'd expect the given probabilities to move in the direction of whichever candidate is leading. Note that this has nothing to do with the empirical statement about whether polling averages move or "break" toward the incumbent or not. It's merely a general statement about the nature of probability, and a recognition that, though plenty of time still remains, another minute has ticked off the clock in the Democrats' drive to retain control of the Senate.
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