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

Never. It only happened in the storybooks. In reality, Democrats have always been and remain the conservative party. They believe in retaining power for themselves and the rarely want to break the status quo. In many ways, the Republikans are technically the more liberal party, favoring adherence to “free market” orthodoxy at the expense of social stability. They just happen to be liberals without the “good” qualities, like respect for human rights or civil liberties.

OTOH, Democrats as a party don’t really have an ideology. They will run people with all kinds of ideological/special interest hangups because they just want to win elections. The result is you often find them infighting once they’re in a position of power. In the 1930s, Southern segregationists and Marxists from the upper west side of Manhattan had no trouble voting for Roosevelt together. Nowadays, poor rural blacks and white urban yuppies, who couldn’t have less in common with each other, back Obama.

The whole liberal-left/conservative-right dichotomy is agitprop designed to impoverish people’s political vocabularies, which makes it easy to frame issues the wrong way.

It was a slow and steady change after 1860.

There’s a lot of cross-overs who decide who to vote for based on past voting records, etc… Personally I’m more liberal on social issues and conservative on financial issues.

Unfortunately some people in both parties are pretty far out there.

Talking about “conservative vs liberal” is a one dimensional view of a multi-dimensional political spectrum.

Many people have pointed out that Democrats are often for increased social freedom (like gay rights) and restricted economic freedom, while Republicans are often for increased economic freedom (free trade) and restricted social freedom. If anything, this should be an obvious hint that the political spectrum is not “conservative vs liberal.”

In the 1930s.

Ignoring the question of definitions, there was a time period during which the Democrats began to hold traditionally Republican viewpoints, and vice-versa. This is the swap that you are talking about, when the Democrats went from pro-state’s rights to pro-federal power, and the Republicans went from pro-federal power to pro-state’s rights.

Here’s one interesting article I found.

The problem of definitions can’t simply be ignored. The Republican Party became the conservative party when investigative journalism largely died with massive media conglomerates took over most of the methods of information distribution. Their reporters blithely call Republicans conservatives because their bosses tell them to. And if you repeat a lie loudly enough, and often enough, people begin to believe it is true. In actual fact, wanting to get rid of the core institutions that have served us for 70 years or more and try radical new forms of governance is NOT conservative. It is the exact opposite.

So it’s true that today, the Democratic Party is the true conservative party, and it is grudgingly nudged ahead from time to time by its progressive wing.

How did it get that way? Republicans gravitated to representing the rich and big business. There’s lots of money in that, and the money was attractive as a way to fight Democrats, who had raw numbers of membership on their side after Hoover presided over the Great Depression.

But the problem is it’s hard to win elections with just part of the top 1% voting for you. So Republicans set their sights on finding voting blocks they could attract with promises of pushing their pet cause.

First, the Neo-Cons left the Democratic Party in the 60s when Democrats befriended the war protesters. Seeing a voting block to grab, the Republican Party welcomed them in. Next, when Lyndon Johnson ordered Federal Troops into Selma Alabama to enforce the Civil Rights act, the Dixiecrats split from the Democratic party. Republicans swallowed the solid South. When Democrats defended abortion rights, the Christian Right moved over to a welcoming Republican Party.

So the liberals in the Republican Party (there would have never been a voting rights act and a civil rights act without Republican votes to overcome the Dixiecrats’ opposition) slowly got purged. Today’s Republican Party became the disgusting sack of s**t it is by eating the detritus of the Democratic Party in hopes of consolidating enough disparate special interest groups to be able to pass laws that benefit multinational corporations and the wealthiest Americans.

Call that a brief and ugly history of the last 80 years in American politics.