The tax break that became one of Trump's biggest campaign promises came from an unplanned conversation, not a policy shop — a reminder of how loosely major tax policy can start.

• Trump has told Senate Republicans the idea came from a waitress at a Las Vegas restaurant who complained about how hard and costly it had become to handle taxes on her tips.
• He says he wrote "no tax on tips" on a notepad at the table and asked her what she thought.
• Days later he tried the line at a Vegas rally with no research or policy paper behind it, and the crowd loved it.
• The pitch worked well enough that Kamala Harris adopted the same promise during the campaign.

Outlook: Expect the policy's popularity with tipped workers to keep it alive politically, even as the details of who qualifies stay messy.