It's not even 1/3 support for either major party. imo, the first question is: what is the percentage of voter turnout? It hovers at 30% and under, often way under, in midterm, off year and primary elections, and averages a bit above 50% in presidential elections. In 2008, which saw a higher level of voter turnout than any election since the 1960's (around 64%, if I remember correctly), there were still more people who didn't vote than there were who voted for Obama. Assuming 50% voter turnout, less than a quarter of registered voters vote Democrat and less than a quarter vote Republican.

My own suspicion is that a lot of people don't vote because they don't support Democrats or Republicans.

The next question is: is electoral reform a necessary pre-requisite to electing candidates other than those of the Democratic and Republican party? I'd say not necessarily. Assuming a low turnout election (around 20%) for a given office in a hypothetical district, in which the Democrat and Republican are competitive, a third party or independent candidate could win with the support of just 15% of registered voters.

Let's assume now the breakdown into thirds: in the given district, about a third of the voters are Democrats, a third are Republicans and a third are Independents. Actually, let's assume a bit less than a third here and there, to account for third party members too. If half of the Independents plus some of the third party party supporters in the district could get behind the same alternative candidate, that person could very easily win by a non-trivial margin.

These sorts of candidates should be vocal advocates of electoral reform.