The importance of a single vote -- and of ours
One side or the other is going to get that majority. Ruth Bader Ginsburg, one of "our" justices, is now 80. Antonin Scalia, perhaps the worst of "theirs", is 77. Anthony Kennedy, often a swing vote, is 76. All three will likely retire during the term(s) of President Obama's successor. Imagine how much more secure freedom and progress in this country would become if the replacements for all three were to be chosen by a Democratic President. Now imagine the likely consequences if it were by a Republican.
The point is that a Democrat, any Democrat, will be vastly better on Supreme Court appointments than a Republican, any Republican. Obama is exasperatingly centrist for the taste of many progressives, but Kagan and Sotomayor have been reliable, and it's impossible to imagine McCain or Romney choosing them or anyone remotely like them. Pretty much any plausible Republican in the White House after 2016 would mean three more Scalia types on the Court, stifling progress for decades to come. Pretty much any plausible Democrat in that position would mean we would never again have to worry about another abomination like the VRA ruling. (Besides, think of all the wingnut heads exploding at the sight of a Democratic President, even a moderate, choosing Scalia's replacement -- I want to watch that!)
No matter who our 2016 nominee is, there will be people insisting that he or she is not good enough on one issue or another, and loudly threatening to stay home or throw away their vote on a third candidate. Such people are fools. Don't listen to them. Remember what's at stake, for far longer than just eight years.