Back in the day – roughly the third quarter of the 20th century – observers of American politics debated the wisdom of what seemed to be a Tweedledee-Tweedledum party system. Some thought it was pretty good. In the 1960s, political scientist Robert E. Lane hailed an emerging “politics of consensus in an age of affluence.” Government by agreement and expertise would replace divisive, ideological politics.[1] Famed political columnist James Reston explicitly endorsed Tweedledee-Tweedledum parties that disputed only the details of the emerging welfare state. He counseled Republicans that their best route to success was “not by moving to the right and exaggerating the differences” with the Democrats, but by showing that they “can administer [liberal policies] more efficiently.”[2]

(Source)
Others thought the similarity in positions was terrible for democracy. Conservatives demanded A Choice, Not an Echo. Leftists bemoaned a “choice of a tweedledee as against a tweedledum” and liberals’ timidity to go to a third party. [3] In 1950, the American Political Science Association complained (pdf) that the parties’ differences were too poorly defined against one another and that they were insufficiently cohesive. Beware of what you ask for.
As is well-known, the political positions of the two parties have divided sharply since those days. This animation 
[3] shows visually how members of the House separated out on a left-right dimension from roughly 1950 to 2000. Most of the shift has been due to the GOP moving right, exactly opposite to James Reston’s recommendation. Early analyses of this ideological polarization stressed that it seemed to be exclusive to politicians and the politically active, that average Americans were not drawn into this ideological fight. Recent work suggests that, while average Americans have still not gotten more ideological, they have become more tightly loyal to their parties as the parties have become more distinct. Party identification has almost become almost tribal. (See this earlier post.) Three new studies underline the power of party loyalty.











