In the classic Katherine Hepburn-Spencer Tracy film State of the Union one of the characters, a veteran Republican politician played by Adolf Menjou, defined the difference between the country’s two major parties thusly, “They’re in and we’re out.” That cynical view summed up the way party hacks viewed the electoral process. The only goal was to win; ideology, principle and policies were secondary considerations at best. American politics has come a long way since the era of bosses and smoke-filled rooms that were essential to that story, loosely based on the rise of 1940 GOP presidential candidate Wendell Willkie. Pundits routinely tell us that we now live in an era when pure partisanship disconnected from ideology is on the wane. The civil war that threatens to tear apart contemporary Republicans, as Tea Party activists seek to slay the dragon of the GOP “establishment,” is an example of just how different things are today.

But not, apparently, in the Democratic Party. As today’s Politico story about Kentucky Democrats plotting to unseat Senate Minority Leader Mitch McConnell illustrates, some of the most liberal groups and donors in the country are putting aside any scruples about their most closely held principles in pursuit of winning nothing more than an election. As they have in more instances than you can count in the last decade, liberals are playing by the old rules of politics while their opponents are doing something entirely different. While they are opening themselves up for criticism from their base, it appears that a party once known as the epitome of anarchy is focused on one thing and one thing only: holding onto Congress.

As Politico notes, Kentucky Secretary of State Alison Lundergan Grimes is no favorite of environmentalists. The Democrat’s likely candidate against McConnell is a supporter of the coal industry and a critic of the Obama administration’s Environmental Protection Agency’s efforts to regulate the fossil fuel industry out of existence. But that isn’t stopping leading “climate change activists” and Democratic donors from lining up to help her with their wallets open.

“It is far better to win the Senate than have every senator on the same page,” [Susie Tompkins] Buell said in an email after an October fundraiser she and her husband, Mark, held for Grimes at their California home. “We can’t always be idealistic. Practicality is the political reality.”

Adolf Menjou couldn’t have put it any better.

For decades, the Democratic Party was wracked by dissension as liberal ideologues sought to purge conservatives from their ranks. Their efforts were largely successful, as the last of the Scoop Jackson Democrats have now left the Senate and the ranks of the Blue Dogs in the House have been thinned to a precious few. While Republicans were eliminating their liberal wing too, the left’s ascendency on one side of the aisle helped pave the way for the GOP revival that ended a half-century of unchallenged Democratic control of Congress. But when faced with a choice between winning an election and purifying their party of any remnants of centrism, liberals seemed to have learned their lesson. As they did in Pennsylvania when they backed a pro-life, pro-gun Democrat in Bob Casey in order to unseat Rick Santorum, liberal donors have their eye on the big prize and are resisting the impulse to nominate more ideologically compatible candidates in favor of someone who can help increase the size of the Democratic caucus in the Capitol.

This wouldn’t be important except for the fact that conservatives are heading in the opposite direction. Across the nation, Tea Partiers are more focused on ending the careers of Republicans that are insufficiently conservative than they are on defeating Democrats and say, making Harry Reid the minority leader rather than the man in charge of the majority. It’s hard not to sympathize with those who are tired of politics as usual and those who waffle rather than take strong stands on the issues. The choice between principle and winning is also not always so clear-cut, as some Tea Party challengers are good candidates and some establishment favorites are duds. But the main point here is that if one of the parties is only concerned with winning and much of their opposition is more interested in something else, you don’t need to be a master prognosticator to know which side is more likely to win.

In real life, politics is not a Frank Capra film where the honest good guys always triumph in the end. Assembling a congressional majority requires compromises and living with candidates that don’t always meet ideological litmus tests but give parties a better chance to win. It may be that in 2013, the answer to the question about the difference between the parties isn’t who’s out and who’s in but which one understands that basic fact of political life.