President Donald Trump and the GOP haven't thought through the details of their tax plan, and must still find a way to pay for it, CNBC's Jim Cramer said Wednesday.

House Republicans, in consulting with Trump, postponed the release of their tax plan by one day until Thursday after disagreements emerged on certain details of the plan.

Cramer suggested that some conservative Republicans would not support the tax plan until it included a concrete way to pay for it.

"You have all these rigid Republicans in the House whom I think are going to veto anything that doesn't have something to pay for it," Cramer said on "Squawk on the Street."

"I just think they haven't thought this through," he said. "Give me a couple pieces of substance. I mean, honestly, does anyone have any great knowledge of what's in there?"

Americans may not even get real a tax reform bill, Cramer said. "I think maybe you get some repatriation, maybe some tinkering with the brackets. But no, because they don't have what to pay for it yet."

Additionally, there are also several U.S. companies that "don't expect much" from the tax plan, Cramer said. The blueprint for the tax plan calls for cutting the corporate tax rate.

"They just say, 'Look, it's not in our numbers. We're not making a budget for it,'" he said.

If Republicans want to attract Democrats, they have to address concerns that the tax plan would largely benefit the rich, Cramer said.

"A tax cut for the rich is just horrendous for the Democrats," he said. "They can't do it."