I think it's best to look at Liberalism's roots in Communism. FDR, Truman, and others were all fans of Communism, if not the Soviets. The Liberals from the 1920s on were all convinced that Communism was the way to go... that everyone should share equally both the pain and the gain from their country. But then human nature had to ruin it's ugly head and ruin the party.
The Liberals had their heydays in the 1950s though the 70s when America was prosperous and we were more open to sharing. Then along comes the high-water mark with the election of Jimmy Carter and he falls all over himself to give the country away, and the economy suffers. Then along comes Ronald Reagan, affirmative action takes hold, and suddenly all Americans are doing better... not great, but better, and doing it through capitalism not communism.
Along comes the 90s, and the great experiment of communism if undeniably proven a failure. What's left for the Liberals? To survive, they become a federation of different special interest groups... Gays, Environmentalists, Animal Rights folks, Pro-Abortion folks, Anti-Death Penalty folks, etc, and things start to go downhill as many of these groups aren't very popular with the American mainstream, and in many cases they are diametrically opposed to one another. They try to stay alive by painting one crisis after another in terms of minority issues, but thanks to affirmative action, more and more blacks are "movin' on up" and becoming Republicans. Notice that "The Black Vote" only gets treated as a seperate entity in the poor cities, not the rich suburbs. Bill Clinton screws over working people with his trade policies, and the unions, which haven't been externally effective for years, erode even further. The only plus side is that welfare is on the decline, but everyone with half a brain knows all Cinton did was put his name on a Republican effort and bill.
So now we're in the new century, and what's happening with Liberals? More people are poor than ever before, but more people are if not wealthy then at least comfortable, so aside from lots of propoganda and demonizing of the rich, which plays well with the poor and the stupid and that's about it, money is pretty much out of the picture. Bush has pushed accountability and funding of schools to the point where complaining about the quality of education places you in danger of directly attacking the weak leg of that stool - teachers. Most of the Liberal leaders of today were spoon fed an anti-military view by their Soviet backers in the 70s, and have a well-established track history of being anti-defense, anti-military, that can't be overcome by a few campaign speeches.
That leaves Liberals with few real central/core values besides abortion, and while most all Abortion advocates are Democrats, not all Democrats are pro abortion. So, Liberals have shown that they can't tend to the needs of the poor, they can't defend the country, they can't build an economy, they can't provide healthcare, and they're pro-abortion, and they believe in a failed social-economic system. And while several key states were split politically in the last election, many were not and were heavilly Republican/Conservative. And many states that should have been heavilly Democratic were shiting to the Right.
Now is just not a good time to be a Liberal.