Both conviction and strategy propelled the Democratic change. As conservative Democrats have become conservative Republicans, there were fewer and fewer voters and politicians inside their coalition inclined toward restraint.

Many Democrats became convinced, as well, that a strong pro-immigration stand could help them win elections because it would appeal to Hispanics, a growing share of the electorate. The white working-class voters whom that stand might alienate were, in contrast, a shrinking share.

At the same time, support for immigration became more and more associated with cherished values such as tolerance, openness and opposition to racism; support for restrictions on it, with intolerance, closed-mindedness and bigotry. The familiar dynamic of polarization also took hold: The issue became a mark of distinction between the parties, and a desire to crack down on illegal immigration and cut legal immigration became something Democrats linked with their partisan enemies.