What's the tax cut done by Reagan got to do with Democratic party realignment?

Richard Darman thought hard about the Reagan administration’s 1981 success in enacting a radical tax cut despite opposition from a Congress controlled by Democrats. And he listed out 10 points, one of which is Reagan’s personal and political appeal among more conservative Democrats worried about a possible party realignment. (Essence of Decision, p.264)
I don’t understand what the tax cut has got to do with Democratic party realignment? Please help me. Thank you.

The Reagan era was well into the era of re-alignment, which began more or less with Richard Nixon. The re-alignment is pretty much completed by 2000, if not earlier.

Before Nixon, the southern states were pretty much one-party states controlled by conservative Democrats. The Republican Party almost never won anything.

Southern Democrats allied with conservative Republicans in the north to create the Conservative Coalition, which opposed most progressive reforms from the late 19th century onward.

Roosevelt, Kennedy, and Johnson represented the liberal movement of the national Democratic Party. By 1964 and 1965, on a National basis, Democrats were leading the progressive movement. But even then both major parties had substantial numbers of both liberals and conservatives.

Now, the realignment is complete, Southern conservatives, indeed, conservatives everywhere, are nearly all Republicans, and almost no Republicans are liberal. The realignment that conservative Democrats feared in 1981 is full and complete now.

I can’t say without reading the source itself why that played into the tax cuts. I’d be guessing. But if I had to guess it may have been the idea that conservative Democrats hoped to stem the movement of conservative voters to the Republicans party by appealing to nominally conservative ideals, like low taxes. Or to generally popular policies, rather, since conservatives never really believed in low taxes except as a rhetorical matter.

Does the “party realignment” worry at the time in 1981 mean possibly more liberals would join Democrats to outnumber conservative Democrats, which would make more conservative voters side with Republicans?
Thank you.

By 1981, liberals were pretty much already settled into the Democratic Party. The Southern conservative Democratic elected officials were seeking to stem the tide of conservative voters becoming Republicans.

I don’t believe this is as straightforward as you make it. At that time and for a decade or more later, there were liberal Republicans in the mold of Nelson Rockefeller, Lincoln Chafee, Jim Jeffords, etc.

I think what happened previously was that more politicians were split on the economic/social divide. A social liberal but economic conservative had a home in the Republican Party. A social conservative but economic liberal had a home in the Democratic Party.

The economics of the 1970s and the Reagan Revolution convinced a lot of people (whether rightly or wrongly, and for another thread) that the old “tax the rich” wasn’t so good and became more economically conservative. So if you had a social conservative Democrat who was going through that switch, he didn’t align at all with the Democrats and just remained one until his state became sufficiently Republican to be elected in that party.

The social aspect was a little different as the Republicans built their resurgence on social issues and its newly found base in the South pretty much demanded that its candidates be conservative on social issues. Liberal Republicans found themselves in the position of Fred Flintstone’s cat. More and more they were outcasts in their own party and they typically lived in states where they could be elected as a Democrat anyways.

So, through these two events, we now have the polarized system that we have where it is a rare politician indeed who does not fall in line to the cookie cutter model of what his party “is.”