That's the two-party, least worst candidate theory. It is only self fulfilling because it's never been successfully challenged...wait, wasn't the Republican party a "third party" that became one of the big two after it elected Lincoln?
The Republican Party was meteoric by today's standards. They were organized on February 28, 1854, held their first convention on July 6, and elected forty-four Representatives and fifteen Senators in November (19% of the House; 25% of the Senate). In the 1854 and '58 elections, with the help of some other parties, they denied majority control of the House to the Democrats. They unsuccessfully ran John Fremont for President in 1856. When they elected Lincoln as President in 1860, they took majority control of the House and the Senate. By the time Lincoln began his first term as President, the party was in place and in power. Prior to this period it was the Whigs and the Democratic parties.