As near as I can tell it began in 1995, when Republicans won control of the House of Representatives for the first time in 40 years and captured a Senate majority.
By “it” I mean the accusation that Republicans and especially conservatives (not always the same) are “divisive.” One reads and hears this from every media outlet and Democrats in Congress. President Trump is dividing America. Talk radio is divisive. The right wing is undermining our “unity.”