As Michael Medved points out the Republicans were never really a third party, the Whigs they replaced was only 23 years old when it disappeared in 1856.
” 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.”
It’s also worth noting that the Whigs collapsed in much the same way the GOP is collapsing today.
Whigs they replaced was only 23 years old when it disappeared in 1856.
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I am not an American political historian (I am a medievalist), but from what I remember when the Republicans were founded there were only the Whigs and Democrats (making the Repubs a “third party”. With the rise of the Republican party, the Whigs disappeared, thus leaving us with only 2 parties again.