Republicans are maintaining, and maybe expanding, their edge in battle for Senate

The Upshot, the New York Times’ successor to Nate Silver’s 538, gives the Republicans a 60 percent chance of winning a majority in the Senate. This assessment is similar to the one Silver rendered in June. Both assessments are based on poll averages.

However, the research firm YouGov, in partnership with The New York Times and CBS News, has just released poll results that seem more favorable to the GOP. The results come from an online panel of more than 100,000 respondents nationwide. Thus, in most states the sample size tends to be larger than the typical single poll, though not as large as in a poll average.

Note, however, that the YouGov poll lacks another major virtue of poll averages — methodological diversity — and suffers from whatever limitations inhere in internet polling. Nate Cohn of the Upshot discusses these issues here.

The YouGov poll has the Republican ahead in five hotly contested states where the seat at stake is held by the Democrats — Arkansas, Louisiana, North Carolina, Iowa, and Michigan. Adding these five to the three states where a Republican pickup is highly likely — Montana, South Dakota, and West Virginia — brings to eight the total of states where a Republican gain is more likely than not. (No Democrat is leading in a race involving a Republican-held seat).

This compares favorably to the poll average data used by the Upshot to calculate Republican chances of winning a Senate majority. These poll averages show the Republicans gaining seats only in Louisiana, Arkansas, Montana, South Dakota, and West Virginia. The 60 probability of Republicans gaining more than five seats arises from the fact that the Republican challenger is only slightly behind in a fairly large number of other states (notably Iowa, Colorado, Michigan, and Alaska).

Accordingly, if one were to use the YouGov poll to estimate the probability of Republicans winning the Senate, their chances might well be better than 60 percent.

The YouGov poll arguably overstates Republican chances of picking up the Michigan seat. Most polls have Democrat Gary Peters leading Republican Terri Lynn Land.

At the same time, I think YouGov understates Republican prospects in Alaska. For one thing, the Republicans haven’t yet settled on a nominee. For another, the poll encompasses only 452 Alaskans.

Other than these two states, the YouGov results seem quite plausible to me.

Many of the margins are razor thin and, of course, much can happen in the next three months. But burdened by their unpopular president, the Democrats are running uphill, and there is good reason for Republicans to be optimistic about their Senate prospects.

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