Largely because of messaging. Someone tells you it's good over and over and over again. Sooner or later you're going to start believing them.

I think tax cuts are going to be one of the main things on people's heads come November. Increase someone's take home pay by $1200, they are definitely gonna remember. Pointing out that rich people got more isn't going to upset too many people as long as they get theirs

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Mr. Cohen cautioned that the bill still was not particularly popular, and opposition among Democrats remained strong. Still, support has grown even among Democrats, from 8 percent just before the bill passed in December to 19 percent this month. For Democrats, Mr. Cohen said, running on opposition to the bill has become more of a political gamble.

“It’s less of a sure bet than it seemed in December,” he said. “This isn’t a problem yet for Democrats, but the movement isn’t a positive one.”
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https://mobile.nytimes.com/2018/02/19/business/economy/tax-overhaul-survey.html