President Trump stunned members of Congress, and his own Treasury Department, when he said last week that he would unveil a plan on Wednesday to deliver “maybe the biggest tax cut we’ve ever had.”

While Mr. Trump’s plan is expected to be only a broad outline of his vision for overhauling the tax code, lawmakers, lobbyists and investors will be parsing every word. The key questions will be whether Mr. Trump is aligned with House Republicans, whether he is keeping his campaign promises and, of course, whether his proposal has any chance of turning into law.

Here’s what to watch for:

Just rate cuts, or real change?

Veterans of previous tax fights like to say that rewriting the tax code is the most complicated legislative challenge there is, creating winners and losers across the entire economy.

During the presidential campaign, Mr. Trump called for cutting the top individual tax rate from 39.6 percent to 25 percent. He wanted fewer tax brackets and deductions to simplify the code.