Why do many Democrats fall for "moderate" Republicans?

[Southern Senator North Carolina Yankee]

Like has been said above, the national alignment is driven by socio-cultural issues.

There are a lot of high end or middle class people who vote Democratic because they are secular and liberal on social issues, but they still don't like taxes. These people would have been Republicans in say the 1970's when the divisions were more class based then anything else. These people are in especially high numbers in the Northeast, where Democratic dominance since 1992 was a result of the 1992 campaign, and the backlash against Republicans on social issues. Since Republicans had also raised taxes, they got the worst of both worlds and defected on masse over the coming decades. The wars and now Trump have only further hardened this alignment.

It is rather similar to how a bunch of pro-business types in the South ended up as Democrats for a long time because the alignment was based on civil war legacy/social issues. It took the New Deal to break this alignment down and replace it with a largely class one.