Resolved: That the Constitution confers upon Congress sovereign powers over the Territories of the United States for their government; and that in the exercise of this power, it is both the right and the imperative duty of Congress to prohibit in the Territories those twin relics of barbarism - Polygamy, and Slavery.
From the Republican Platform of 1856, the founding document of the Republican party.
Horace Greeley, one of the founders of the Republican party
Philadelphia today is a very Democratic city but this wasn't always so. It was once a solidly Republican town. The first Republican Party Convention was held here in 1856 and Philadelphia has hosted the convention a total of six times, most recently in 2000.
Viewed through the long lens of history, the dramatic changes the Republican party has undergone in its century and a half of existence are striking. It was created specifically as a political instrument to oppose slavery by northern, urban abolitionists.
The earliest representation of the Republican party as an elephant, appeared in an 1874 Harper's Weekly cartoon by Thomas Nast
In contrast to its historical roots, today's GOP is strongest in the south and in rural areas. Most significantly, a cornerstone of modern Republican political philosophy is limited government. But in 1856, it was Republicans calling for the exercise of federal authority to halt the expansion of slavery, while southern Democrats defended their "peculiar institution" on the principle of state's rights.
The evolution of our two major political parties into the very different entitites they are today is all part of the ongoing story of the USA. But like the history of our nation itself, the history of the GOP starts in Philadelphia.