In the current environment of low interest rates, issuing longer-term debt could save taxpayers money in the long run by locking in reduced borrowing costs. The long bonds would also likely appeal to long-term investors such as pension funds and insurers, and they could be used to fund infrastructure projects. However, there are questions about investor appetite for such assets and whether they could prove to be disruptive to markets.

Interest in the idea appeared to fizzle since Mr. Mnuchin first examined it in 2017. But Mr. Trump seized on the concept over the summer after the yield curve in the bond market “inverted,” spurring speculation that a recession could be coming. An inverted yield curve occurs when longer-term interest rates fall below shorter-term interest rates, and has historically been a warning sign that a recession could be on the way.

“I told Secretary Mnuchin that this is a great time to refinance our bonds, or some of our bonds,” Mr. Trump said in August.

The president addressed the issue again on Wednesday when he called on the Federal Reserve to cut interest rates and said on Twitter that “we should then start to refinance our debt.”

Mr. Trump, who often fears that the United States is being outdone by other countries, might have taken notice that Britain, Canada and Italy have sold 50-year bonds in recent years, and 100-year debt has been issued in countries like Mexico and Belgium.

As the outlook for global economic growth darkened this year, yields on long-term Treasury debt have tumbled sharply. The yield — the interest rate that investors earn — on the 10-year note fell to 1.46 percent on Sept. 4 from more than 3.20 percent in November 2018. Around the same time, yields on the 30-year Treasury bond dropped below 2 percent for the first time, touching a low of 1.94 percent on Sept. 3, according to data provider FactSet. Even though yields have risen somewhat since, they remain significantly lower than where they were last year.

When bond yields fall, prices rise. So the recent drop in yields suggests strong demand for the longest-maturity debt the Treasury currently issues.