When did the Republican party become the conservative party, and the Democratic party become the liberal party?

About 50 some years ago, when I was first getting involved in politics, there were Democrats and Republicans,(and the occasional independent) and within each party there was a whole spectrum, ranging from conservative to liberal. (Yes, Virginia, there really were Conservative Democrats and Liberal Republicans!) Over the past few decades, however, there seems to have been a polarization of the political scene to the extent that the spectrum is divided by party. The Republicans seem to have migrated to the “Moderately-to-Extremely-Conservative” side of the spectrum, while the Democrats cover the “Somewhat-to-Moderately-Liberal” side.

IMHO, there are few, if any, true progressives in congress. The term has become synonymous with liberal.