But for many of those years, the concern was that the parties were too much alike and philosophically undefined.
For instance, if you said the word "Democrat" in the 1950s, you might be describing a Southern segregationist or a left-wing Northeasterner. Republicans for decades were united primarily by their views on economic issues, tolerating a broad range of opinion on the social ones and on national security.
Now, the opposite is true. Party labels have become a shorthand for a rigid ideological dividing line — Democrats to the left, and Republicans to the right.
And the clout of the parties has receded even more quickly in recent years, thanks to the 2002 McCain-Feingold campaign finance law that cut off their access to unregulated contributions known as "soft money," and the Supreme Court's 2010 decision in Citizens United v. Federal Election Commission, which opened up the spigot for fund to flow to outside groups.
"The law isn't the explanation for the weaknesses of the parties, but the law has accelerated their struggles," said one top Democratic National Committee official, speaking on the condition of anonymity because of the sensitivity of the subject.
Political parties increasingly are being outmatched in resources and organization by special-interest groups or those, such as the tea party, devoted more to furthering a cause than achieving electoral victory.
As a result, the parties are no longer as able as they once were to protect their incumbents from ideologically-driven primary challenges, to define their messages or even to keep up with technology.
Last year, for instance, the parties spent a total of $228 million on independent efforts to boost their candidates, primarily with ads. That was well under half the $631 million spent by the so-called "super PACs" that have sprung up since the Citizens United decision, according to figures compiled by the Center for Responsive Politics.