There May Be Only Two

The Democratic and Republican parties do not hold their virtual duopoly on elected offices in the United States because of a corresponding duopoly of public support - there are many people who choose to support neither the Democrats nor the Republicans, and do not vote for their candidates. Instead, the two principal parties maintain a lock on elections by maintaining a lock on the perception of viability.

Despite what some Republican boosters would tell you, Senator Bernie Sanders is not running for the Democratic Party nomination for President of the United States because his brand of Social Democracy is a natural fit for the traditional Democratic voter. Over the years, Senator Sanders has made a number of unflattering statements about the party:

“You don’t change the system from within the Democratic Party.”

“My own feeling is that the Democratic Party is ideologically bankrupt.”

“We have to ask ourselves, ‘Why should we work within the Democratic Party if we don’t agree with anything the Democratic Party says?’”
Can Bernie Sanders Win the Love of a Party He Scorns?

Instead, he joined the Democrats because he understood that it was the only way to make a serious run for the White House.

Sanders said he thought about running as independent in the first place, but decided not to.

“If we were serious about winning this election, which is always my intention from day one, I thought we could and I hope that we will. I had to do it within the Democratic primary caucus process,” he said.
Bernie Sanders on why he won’t run as an independent

The alternative is to be relegated to the “Third Party” ghetto, where candidates can, and do, build loyal supporter bases, but next to nothing in terms of either media coverage, donations or the sort of broad support needed to be taken seriously.

As long as the public as a whole demands that viable candidates only come from the Democrats and Republicans, this situation is not going to change.