Complex sentence: Earlier that year the GOP keeping on the path set by Klansman President Harding had promised not to speak against the KKK at the RNC let alone adopt an anti-Klan platform now if the Klan could cow the Democrats it would have complete control of both parties.
Split into simpler sentences: Earlier that year the GOP keeping on the path set by Klansman President Harding had promised not to speak against the KKK at the Republican National Convention (RNC), let alone adopt an anti-Klan platform. Now if the Klan could cow the Democrats it would have complete control of both parties.