The GOP’s strong 2016 election showing raises a crucial question: Do Republicans have any chance of netting eight Senate seats — and a filibuster-proof majority — in 2018?

The upcoming Senate class is unusually unbalanced. Only eight Republican Senate seats are up for election in 2018, compared with 25 Democratic seats (including two independents who caucus with the Democrats). Ten of those Democratic seats are in states carried by Donald Trump.

By any measure, Democrats are on the defensive in the next fight for Senate control. A three-seat Democratic midterm gain, which would give the party a majority, looks virtually impossible given the seats up this cycle.

A net change of eight seats would be large by historical standards but not unprecedented. Swings of at least eight Senate seats have occurred in four of the last 17 midterm elections — 1958, 1986, 1994 and 2014 — and in six of the last 34 elections (going back to 1950).

The problem for Republicans is that these big Senate swings have always happened against the sitting president’s party. The sole exception, since the direct election of senators, occurred in 1934, when President Franklin Roosevelt’s party gained 10 Senate seats. Two years earlier, when Roosevelt won a landslide presidential victory, his party gained a dozen Senate seats. ....