What issue did the Republican Party rally around in 1854?
Slavery in the new territories.
The Republican Party was formed around a single issue: They opposed the expansion of slavery in the new territories acquired during the Mexican War.
They were not initially abolitionists. Their earliest members recognized hat the Constitution explicitly allowed the practice and they did not seed to ban it in the (Southern) slave states where it was already legal. The did, however, oppose expanding the practice into the new Southwest.