In a piece at the New York Times Nate Cohn reviews some of the possible outcomes in the midterm elections. I recommend you read the whole thing but here is the kernel of his piece:
The possibility that the Kavanaugh nomination is helping Republicans in Republican-leaning areas is important because the fight for control of both the House and the Senate will be determined largely in Republican-leaning areas. This simple fact has always been the G.O.P.’s biggest advantage. If the electorate is polarized along the lines of recent presidential elections, as it was during the Obama presidency, Republicans could hold down their losses considerably.
Democrats have been considered clear favorites in the fight for House control because polls and special election results have made it seem that the electorate wouldn’t be so polarized, allowing them to compete in many Republican-leaning districts. But if Democrats can’t break through and actually carry the many Republican-leaning districts they’ve put into play, Republicans could stay highly competitive in the fight for House control and even survive a wave election.
A lot of the commentary including mine has focused on the likely outcomes and Mr. Cohn’s is no exception. The greatest likelihoods are either that the Democrats eke out a narrow victory in the House and fail to take the Senate (which I’ve been predicting for some time), the Democrats receive a narrow House majority and an even narrower majority in the Senate, or that Democrats fall short of taking control of either house of Congress.
Let’s consider some of the less likely alternatives.
The Democrats pick up large majorities in both houses of Congress
For Democrats to pick up as large a majority in the House as the Republicans have now they’d need to net 50 seats or more. That has happened twice in midterm elections in the post-war period and in both cases it was the Republicans who accomplished the feat. It’s not beyond the realm of possibility but it’s very unlikely.
The Republicans pick up seats in both houses of Congress
By my reckoning the president’s party picking up seats in both houses has happened exactly once since the end of World War II—in 2002—and again it was the Republicans who did it. Given the present polls and President Trump’s approval rating, I think this is quite unlikely. At the time of the 2002 midterms President Bush’s approval rating was 68%, something hard to imagine now.
The Democrats pick up seats in the House and lose seats in the Senate
The president’s party losing seats in the House and picking them up in the Senate has happened three times in the post-war period: in 1962, 1970, and 1982. This is not an outcome that Democrats should wish for. In each case the House gains were small—not nearly enough for a majority—and Senate gains would strengthen the Republicans’ hand. IMO this particular outcome is nearly as likely as the likely outcomes of the prevailing wisdom.