When Donald Trump's policy adviser Stephen Miller stepped into the White House briefing room Wednesday to defend a plan for reducing levels of legal immigration, Jim Acosta of CNN was aghast and let everyone know it.

Put aside that Acosta believed it was his role to argue one side of a hot-button issue. The exchange illustrated how advocates of high levels of immigration are often the ones who — despite their self-image as the rational bulwark against runaway populism — rely on an ignorant emotionalism to make their case.

At issue is the bill sponsored by Republican Sens. Tom Cotton of Arkansas and David Perdue of Georgia to halve legal immigration. The legislation would scale back so-called chain migration — immigrants bringing relatives, who bring more relatives — and institute a merit-based system for green cards based on ability to speak English, educational attainment and job skills.