U.S. President Barack Obama speaks about immigration reform from the Rose Garden of the White House in Washington

Democrats are apprehensive about this year’s midterm elections.

They should be.

Every indicator points to Republican gains in Congress. Two reasons are well known: President Barack Obama’s unpopularity and the historical record of midterm elections, when the president’s party almost always loses seats.

The third major reason is the two-four-six rule. Those are the different base years for different offices: two years for the House of Representatives, four years for most governors, six years for the Senate. These base years dictate how vulnerable each party is.

Here’s how it works: House members last faced the voters two years ago, in 2012, when Obama won re-election. With Obama’s strong voter turn-out, Democrats gained eight House seats. In the 2014 midterms, however, with their expected older and whiter electorate and Obama’s low poll numbers, Democrats are facing a tough November.

McConnell, Reid and Boehner lock arms and sing during a ceremony to award posthumously the Congressional Gold Medal to civil rights leaders Martin Luther King Jr. and Coretta Scott King, in WashingtonTurning to the Senate, 33 of the 36 seats being fought over in November were last up in 2008 — when Obama first took the White House in a stunning victory. Democrats picked up eight Senate seats.  (The other three Senate races are special elections for partial terms.)

When a party picks up seats, as Democrats did in the Senate in 2008 and the House in 2012, the gains tend to occur in swing states and districts. The party has difficulty defending those seats the next time they are on the ballot. That may be particularly true now since the president’s popularity has dropped to 41 percent and shows no signs of recovering.