Why was the democratic party, as the only remaining national party, unable to avoid the growing sectional polarization of the 1850s?

There was a backlash to the liberal policies that were prevailing at that time. There was a movement to a more conservative and balanced approach to government, A new ideology was born, and eventually graduated into the Republican Party. A socialist movement also gained popularity due to this backlash.

The tried to make the government smaller by making the government smaller