[First,] Democratic observers point out that their voters are not evenly distributed throughout the country. Instead, Democrats tend to be packed into urban areas, which mean that they are likely to underperform a very close popular vote, at least slightly.
The recent round of redistricting only added to Democrats' problems, as Republicans protected their incumbents and took districts off the table.
Second, history suggests that it will be difficult for Democrats to regain the House next month.
Given that the chamber didn't flip for more than 40 years, regardless of the kind of election, it may not be meaningful to note that the House hasn't changed party in a presidential year since 1952 (or since 1948 when a sitting president beat back a challenger), as Paul Kane of the Washington Post has observed.
Still, it's hard to ignore the fact that in four of the last five times a sitting president was re-elected (2004, 1996, 1972 and 1956), his party gained or lost a dozen seats or fewer. In the other case, in 1984, the president's party gained 16 seats. In other words, election years when a sitting president is seeking another term generally haven't produced dramatic swings in the House.
What would explain the relatively small changes in the House in presidential re-election years, especially in light of the huge swings in midterms?Moreover, a small number of voters may want the opposite party to control one chamber of Congress in order to serve as a check on the president.