Almost all Republicans are conservatives, at the voter and office holder level. Only about half of Democrats are liberal, at both levels. Thus, the conservative "base" has a lot more leverage over "its" party than we, the liberal "base," have over "ours."
Although freemansfarm votes for Democrats, he makes the ultimate argument to not vote for them in the three sentences I quoted. The Democratic Party has no reason to do anything liberals want. Suppose you're a Democratic Party leader and have two options on an issue: Option A, which liberals like and non-liberals hate, and Option B, which liberals hate, and non-liberals like. Which option would you go with?
If you care about winning elections, you'll go with Option B. If you go with Option A, the non-liberals can go to the Republicans, which means you lose. By going with Option B, you get all the Democrats. The non-liberals stay with you because you're doing what they want. The liberals will be upset, but they'll vote for you. They won't vote Republican. They won't vote for a third-party candidate or not vote at all because they fear Republicans being in power. Democrats get the most votes by ignoring what liberals want and doing what non-liberals want.
Politicians care about one thing: getting elected. Democratic politicians currently increase their chances of getting elected by ignoring liberals and doing what non-liberals want. They will continue to do this as long as liberals continue to vote for them unconditionally. The only way for liberals to get Democratic politicians to enact liberal policies is to stop voting for Democrats.
