I used to believe the Republican party brought much needed conservatism to the table. There were reasonable concerns that the Democratic party was too heavy handed with implementing morality and over reaching laws. The Democratic party has mostly been in the right side of social permissivness since then and the Republican party has gone fucking crazy Reactionary which they have rebranded as "Conservative". It has become an intersection dynamic where the Democratic party has become a coalition of progressives and conservatives, who just to want to keep the rights they have. Unfortunately there are many "Team R" fans that don't recognize that their party no longer represents them.

So short answer, the Democratic party has already absorbed the strengths of the Republican party.