It's not that we tolerate it, it's that our voting populace is largely single issue voters and don't really care about whether the next president wants to remove the Patriot Act. They care about social issues like reproductive rights. Added to mostly binary voting (two party system) means you only have two real choices that are polar opposites on the hot button social issues.
I think in the end it comes down to the way the parties are funded. With state financed parties and regulations to cut costs of campaigns, smaller parties would have better chances to gain traction. The problem is that nobody currently in charge has the slightest interest to do as he would cut of his own monopoly to the financing. Maybe you could start by sneaking in some clever cost-cutting tricks for the campaigns.
It's sad how much advertising plays into elections. It comes down to living in a society where most people are pretty dumb and make voting decisions based on a 15 second commercial and biased journalism.
