He later helped write tax-cut legislation for President Ronald Reagan and advised candidates at all levels of government. His push for sweeping tax cuts to spur rapid economic growth dominated the policy proposals of the 2016 Republican field, and he has continued to advise Mr. Trump informally.

Mr. Laffer was also an architect of the tax-cutting package that former Gov. Sam Brownback pushed through in Kansas. Mr. Laffer and his disciples, including Mr. Moore, insisted that it would jump-start growth and produce a windfall of tax revenue. It instead produced a huge shortfall in the state’s budget, prompting lawmakers to reverse most of the law.

Democrats once sought Mr. Laffer’s advice — he advised Jerry Brown, the former California governor, during his failed 1992 presidential campaign. But in recent decades Democrats have largely moved away from Mr. Laffer, viewing him as an architect of tax handouts to the rich.

Mr. Laffer has heaped praise on Mr. Trump for the tax cuts he signed into law in 2017 and for his steps to reduce federal regulation of business. A longtime proponent of free trade — he has called the North American Free Trade Agreement, enacted under President Bill Clinton, the equivalent of a huge tax cut — and an opponent of tariffs, he has played down his concerns with Mr. Trump’s policy of threatening and enacting tariffs as leverage in trade and immigration disputes with other countries.

In a recent interview, Mr. Laffer said he had told the president what he told everyone about trade policy: “When you look at tariffs, they are very, very bad for the economy.” But he said he believed that Mr. Trump was using tariffs to pressure other countries to open their markets more freely. He called Mr. Trump a master negotiator and said he supported his policies.