Yet the Trump trade was also a wager that the dollar would climb as investment flooded into the United States to exploit fresh growth opportunities.

Those expectations have been overwhelmed by the turbulence of the Trump presidency. Senior government officials have been hired and fired at the pace of a reality television show. Myriad disclosures have intensified questions about whether Mr. Trump’s coterie colluded with Russia to influence the American election. Big parts of his agenda have stalled.

All the while, Mr. Trump has unleashed his signature Twitter rants, sometimes undercutting the positions of his cabinet members and sowing confusion. In the estimation of many economists, the dollar’s fall reflects an assumption that his administration will be hard-pressed to deliver on key goals.

“The potential for serious investment and tax reform and economic growth in the United States is unlikely to be realized,” said Ian Goldin, a former World Bank vice president and now professor of globalization and development at the University of Oxford. “There’s just a mood that gets amplified every time we have a disaster in the White House, or a new tweet.”

Currency values are both volatile and relative. The dollar’s worth must be understood as a reflection of contrasting economic prospects in the United States and other lands.

Mr. Trump’s pro-growth initiatives have been sidelined just as the Federal Reserve has lifted interest rates, constraining American expansion. At the same time, Europe — long a morose topic in the global economic conversation — has shown encouraging signs of vigor.