Steve Smith sends this graph:

In the political science literature, DW-NOMINATE scores a the most prominent measure of the ideology of members of Congress.  This graph plots the standard deviation in those scores from the 84th through 111th Congresses (basically 1955-2010).  The larger the standard deviation, the more ideological heterogeneity there is in the party.

In the earlier part of this period, the Democrats were clearly more heterogeneous, as one might expect in a party with defined liberal Northern and conservative Southern wings.  But over this period, ideological heterogeneity in the Democratic Party decreases substantially.  By about the 104th Congress (after the “Republican Revolution” of 1994), the parties are equally heterogeneous.   And that has continued to be true.  Via email, Smith says:

There is hardly any difference since start of the Gingrich era (he became Republican whip in 1989).  ”Gingrichism” encouraged a disciplined party to sharpen differences (he thought the public was on his side) and force Democrats to cast more difficult votes (he thought this would put Democrats from conservative districts in danger).  Republicans gave this strategy credit for the 1994 victory.  For detail, see Barbara Sinclair’s Party Wars.

Democrats’ reputation for less discipline, or at least less cohesiveness, was certainly deserved in the period between the late 1930s and 1980s.