WASHINGTON — When Donald J. Trump was crafting his campaign tax plan last year, one of his top economic advisers proposed an idea that would have made it harder for real estate moguls to use mountains of debt to make deals. Mr. Trump, who made his fortune as a property developer and earned the nickname “the king of debt,” scoffed at the suggestion.

“He hated that idea,” said Stephen Moore, the Heritage Foundation economist who counseled Mr. Trump during the campaign.

The proposal to scale back interest deductions for companies did not make the cut.

As President Trump focuses his attention on overhauling America’s tax code, he has considered turning to the other side of the aisle to reach a bipartisan deal. But an obstacle has already surfaced, dimming the chances that a “grand bargain” will be achieved: Democrats are sounding the alarm that reshaping the tax code presents Mr. Trump’s biggest conflict of interest yet.

A tax code overhaul gives Democrats the chance to again bring up Mr. Trump’s refusal to release his tax returns and to press for details of how his business deals are financed. That focus could also affect which tax code items, such as interest deductibility, are included in the overhaul.