The terms "radical" and "conservative" an "leftist" vary in accordance with the political zeitgeist of the era to which they are applied.
The anti-slavery principles which led to the founding of the GOP might have beeen considered "radical" in the 1850s in many quarters. But the Republican Party effectuatated their major achievement of ridding the country of slavery by constitutional means - using the amemdment procedure in the original constitution to bring about the 13th Amendment, and the 14th and 15th amendments to supplement it. So in adhering to constitutional amendment to effect the change it sought, one could describe the early early Republican Party as "conservative." Contrast that to later eras where the Constitution was merely flouted in order to achieve "radical" or "progressive" ends.
so glad to see someone groks it