The two parties, in collusion, have ever sought greater ways to limit outside competition. Ballot access laws, campaign finance laws, and safe districts have done much in this area, but even then third parties are still on the ballot. Occasionally a third party even garners more votes than the winning margin between the two approved candidates. Occasionally a third party even wins some office.
That situation is considered, by those who feel it is their right to rule others, intolerable. They really have no complaint, because in the few instances in which the rules could have applied to the major parties, the rules were waived. In 2004, the Republican Party held its convention rather late in the year to put it as close to September 11th as possible. As such, Bush was named the candidate after the filing deadline had passed in two states that Bush eventually carried. Had the law been enforced, Kerry would have won in 2004. In 2008, both the Republicans and the Democrats failed to file on time in Texas. Had the law been enforced, the Texas electoral votes would have gone to Bob Barr. Theoretically Democrats should have supported enforcing the law as written, since it would have enabled them to sail to an easy victory. In reality the Democratic Party leadership recognized a higher principle, that of maintaining the duopoly, and was willing to sacrifice victory in 2004 and risk sacrificing victory in 2008 to maintain their top positions.
It goes without saying that had a third party filed late, no such exception would be made.
But a way to truly disenfranchise third parties has reared its ugly head and is spreading, one state at a time, through the United States. Currently it is festering in California under the guise of California Proposition 14, the Top Two Primaries Act of 2010.
It purports to fight partisanship by putting all candidates in one primary, without regard for party. Anyone can vote for any parties candidates. The top two vote getters will proceed to the general election.
The minor argument against this proposition is that it will result in two Republicans running against each other in Republican Safe Districts, and two Democrats running against each other in Democrat Safe Districts. This will leave those in the out party having little choice.
The middle argument is that this will entrench incumbents even more. As has been shown in Louisiana, the only incumbent to lose after an approach like this was passed did so because redistricting had that incumbent lose to another incumbent.
The greatest argument is that this will destroy all third party electoral chances. As has been shown in Washington (the state), no third party candidates have been on the general election ballot since that state adopted this measure. California Proposition 14 would even eliminate the protest of a write in candidate, so any third party voter who continues to vote for a third party candidate will not have the vote counted and will have the ballot disqualified.
This measure is exactly what one would want to further entrench the duopoly. It is corruption manifest and must be defeated.