On Election Day (or, with early and absentee voting, during election season), not every citizen who is registered to vote will actually vote. There are a variety of reasons. Some have not put in the time and effort to educate themselves about the people and issues on the ballot. Some don’t believe their vote will make a difference. Some may be confident that their favored candidate(s) will win anyway; some may be fatalistic that their favored candidate(s) will lose anyway. Some may have logistical problems getting to the polls; some may simply forget.

Because of all these factors, it is a given for anyone who has ever been involved in a political campaign that “turning out your voters” is a key to victory. Success hinges not just on persuading a majority of your fellow citizens that you are the best candidate; it also hinges on success in motivating those voters to actually vote.

It should be no surprise that the highest voter turnout generally comes in presidential election years. That is when the media coverage of politics is at its most intense. Even people who pay no attention to local or state legislative races, or even races for Congress or Governor, will generally form an opinion on which candidate should be the next President of the United States, and will make an effort to express that view at the ballot box.

That means, however, that in a non-presidential year, like the 2014 mid-term elections, fewer votes will be cast, and therefor “turning out your voters” is even more crucial.

Anthony Brown learned that the hard way.

Brown has served two terms as Lieutenant Governor of Maryland under Gov. Martin O’Malley, the former mayor of Baltimore. O’Malley is leaving office and is considered a dark horse candidate for the 2016 Democratic presidential nomination. Brown was his designated successor for the governor’s mansion, easily winning the Democratic nomination.

The election should have been a shoo-in for Brown. Maryland is one of the bluest of deep blue states. President Obama carried the state in 2012 with 61% of the vote.

In one of the biggest (and most under-reported) upsets on election night, however, Brown lost to his Republican opponent, Larry Hogan, 51%-47%.

I was curious as to how big a role turnout played in this surprising outcome, so I went back to look at some vote totals I compiled after the 2012 election. (I had written a blog post then about how even in the four states which did not vote to defend the definition of marriage as the union of one man and one woman, the pro-marriage vote had well exceeded the vote received by Republican nominee Mitt Romney.)

Comparing those votes with this year’s governor’s race confirmed the importance of turnout. Although Hogan won in 2014 with 51% of the vote, and Romney lost Maryland badly in 2012 with only 36% of the vote, the raw number of votes Hogan received in victory was only 91% of the number of votes Romney received in defeat.

What does that say about Brown? He received less than half as many votes as President Obama did in 2012—only 792,000 compared to Obama’s 1.6 million.

A similar trend probably prevailed across the country. Masses of Obama voters just stayed home on Election Day—leading to the Republican wave we saw on Election Night.