Just a few months ago Democrats envisioned a “Blue Wave,” gaining 40-50 seats. Today their expectation have been tempered – and for good reason. Since 1946, Democrats have gained an average of 21 seats in the midterms when the president is a Republican. That gain would not be enough for a majority, where the Democrats need to gain 25 seats.

Using election data since 1946, on average the president’s party (Republican or Democratic) loses 21 seats in the first midterm election. But that average masks big swings, from a 63-seat loss in 2010 to an 8-seat gain in 2002. The median loss is just 15 seats. The current partisan makeup is 236-193 with 6 vacancies. RealClearPolitics puts Republicans as favorites in 202 seats, Democrats in 199 seats with 24 tossups.

One thing is for certain, the president’s party struggles in midterm elections because that’s how politics works. Presidents get elected by making a lot of promises to get elected. The result? The president’s voters can never be fully satisfied less than two years into a given presidency. Some of those voters drop out, while the opposition becomes united, not in policy preferences, but just in opposition to the president they didn’t vote for.

Average gains and losses are interesting, but they don’t tell the whole story. For one thing, in three of nine first midterms the president’s party was in the minority. Simply put: The fewer seats you have, the fewer you can lose. The average loss for the president’s party in the majority is 24 seats. However, on two occasions the president benefited from a major foreign policy bump. The successful resolution of the Cuban missile crisis saw President Kennedy’s approval rating jump into the 70s, dampening Democratic losses to merely 4 seats. One year after the September 11 attacks, President Bush’s approval was 68 percent, helping Republicans gain 8 seats. Taking out those two elections leaves us with an average loss of 37 seats.

So, what can we expect for a president whose approval rating is in the mid-40s and whose party is in the majority? Superficially, it looks like 1994 and 2010. In both cases, the incumbent president’s approval was remarkably similar. President Clinton averaged 44 percent approval leading up to Election Day, while President Obama averaged 45 percent. Currently, President Trump is averaging 43 percent (Rassmussen has generally had President Trump between 45 percent and 48 percent, polling likely voters against other pollsters who either poll registered voters or have no voter screen).

It’s unlikely we should we expect Clinton/Obama-sized debacle. There are some important differences that favor the GOP. First, in 2010 the Democrats held 256 seats, 20 more than the current Republican roster — so, more seats to lose. Entering the 1994 elections Democrats had been the majority for 40 years and that election saw the electorate catch up with geographic and demographic changes that had brewing for a long time. Retirements are a problem for the GOP – where 36 Republicans are departing against only 18 Democratic retirements. But that particular headwind was not a problem in 2010 where the GOP also had more retirements (27-17) but still gained 63 seats.

Democrats hope to see a repeat of the 2006 midterms where dissatisfaction with the Iraq War caused Republicans to lose their House majority for the first time since 1994 with a Democratic gain of 31 seats. But that scenario is problematic. At that time George W. Bush’s approval rating was 37 percent in the month prior to Election Day with a disapprove of 58 percent. Trump has a higher approval rating of 43 percent (46 percent Rassmussen) and lower disapproval rating of 53 percent.

One confounding factor is Trump’s odd approval rating track. All presidents get an initial approval rating bump followed by a steady fall. Early presidential approval ratings have averaged 73 percent. All approvals fell at least 15 points within the first two years. Trump started out low, averaging 45 percent, and has barely moved – down to just 43 percent. These numbers indicate a more loyal base for Trump than for past presidents. Trump also polls slightly better in the likely voter Rassmussen poll — and since elections are about who shows up, that could help Republicans.

Both current polling and the historic data point to a very close result on Election Day. The biggest Democratic seat gain since the 1974 Watergate election (+48 seats) was the 2006 Iraq War election. With Trump polling at higher popularity than Bush in 2006, it seems likely that the Democrats will not surpass their 31-seat gain. That leaves very little margin for error to obtain a majority.

In the end, the midterms will depend on turnout. With a volatile electorate, a momentary downdraft on the part of the president could spell doom at the polls — just as a jolt of good news could keep a Republican majority. One thing is for sure, whichever party ends up in the minority is in for an orgy of recriminations and turmoil.

Keith Naughton is a political consultant.

The views and opinions expressed in this commentary are those of the author and do not reflect the official position of The Daily Caller.