What about the Senate?

The United States votes in eight days time in an election that, unless something very big happens in the intervening period, will make Democrat Joe Biden the next president. But his prospects in the job will depend significantly on whether or not his party can also win a majority in the Senate.

The Democrats (counting two independents who caucus with them) currently hold 47 of the 100 seats in the Senate, against 53 Republicans. The vice-president has a casting vote, so assuming that will be Kamala Harris, the Democrats need to pick up three seats for effective control. But one of their 47 was won in a by-election in 2017 in what would usually be a safe Republican state, Alabama, so the real target is a gain of four seats.

To understand how Senate elections work, you should read my preview of the 2018 election and my postmortem on the results. There are three key points:

  • Because a third of the Senate is elected every two years, the key election to make your comparison with is the one six years ago, when the currently retiring senators were elected.
  • Because the Senate is relatively small and senators have high personal profiles, you can’t just assume uniform swing and read the approximate numbers off a pendulum, as you can in the House of Representatives.
  • Because the US is becoming more polarised, those personal votes, while still important, are less so than they used to be – swings are tending more uniform. In 2018, the Democrats’ share of the two-party vote in the Senate (excluding California, where there was no Republican candidate) was almost exactly the same as in the House, 54.3% vs 54.4%. (The official statistics are all here.)

So although we know it’s not completely realistic, let’s start with the assumption of uniform swing and see where that gets us. The first four states on the pendulum from 2014 – a strong Republican year – are North Carolina (requiring a 0.8% swing), Colorado (1.0%), Alaska (1.2%) and Georgia (3.9%). Iowa (4.3%) is close behind, followed by Kansas (5.5%) and Louisiana (5.9%).

There are also two by-elections (the Americans call them “special elections”) for Republican-held seats: Arizona, which requires a swing of 6.4% from its 2016 result, and the other Georgia seat, also dating from 2016 and requiring 7.2%.

Now have a look at what the polls are saying (I’m using FiveThirtyEight’s “polls only” forecast model). As of today, the Democrats are ahead in six Republican-held states: Colorado, Iowa, Maine, North Carolina, and both of the by-elections. Alaska and Kansas are also pretty close, as is the Georgia regular election, and the Democrats are given a fighting chance in Montana, on a 9.0% margin.

There are only two states on this side of the pendulum where the 2014 margin is a particularly bad guide. Louisiana, which should be close, is actually very secure Republican (FiveThirtyEight gives the Democrats only a 6% chance), and Maine, which on paper requires an 18.5% swing, is showing a fairly clear Democrat lead.

But those results are what you’d expect in those states; Louisiana votes solidly Republican in presidential elections, while Maine is somewhat less reliably Democrat. It’s the 2014 results that now look anomalous: Louisiana then had a Democrat incumbent, while Maine has a long-serving Republican senator, Susan Collins, whose luck finally appears to be running out.

So it looks as if the Senate pendulum doesn’t work too badly. And there’s no dispute that this election will show a big swing to the Democrats as compared to 2014: Biden is hovering around 55% of the two-party vote in the polls, compared with 47.0% for the Democrats in the House of Representatives in 2014.*

An 8% uniform swing would bring in eight seats, which is at the high end of expectations. Colorado, Arizona, Maine and North Carolina – probably in that order – look like near-certain gains, which would be enough for control. Assuming, that is, that there are no Democrat losses (except Alabama); the Republicans might indulge some hopes for Michigan (6.9%) and Minnesota (5.4%), but the polls are not encouraging for them.

Beyond those four, the Democrats would still like as many seats as possible. Senators can be idiosyncratic, and it may be that not all of them will support all of Biden’s agenda – Joe Manchin of West Virginia, not up for re-election this year but representing what is otherwise a solidly Republican state, is often mentioned as a doubtful case. The more spares the party can bring in, the better.

So keep a close eye next week on the two Georgia seats, as well as Iowa, Montana, Kansas and Alaska. If the presidential vote is unexpectedly close it will no doubt be hard to focus on anything else, but a couple of Senate seats either way could make a big difference as well.

* Technical note: of course there was no presidential vote in 2014, but why use the House vote rather than the Senate? Because the House vote is a national tally; the Senate vote (which was 46.1%) excludes those states with no senator up for re-election, so doesn’t work as well for comparison with the presidential polls.

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