Trump isn’t president yet. He’s already spooking the Fed.

The Fed’s economic projections, and Chair Jerome Powell’s press conference, spooked the stock market into a sharp selloff. One undeniable takeaway was concern over the president-elect’s policy proposals. This sets up what could become a contentious and destabilizing relationship between the chief executive and the central bank.

Previously, Powell has publicly insisted that there is too much uncertainty in what presidential policies will be pursued, and what their impacts will be, for the Fed to start factoring them into its decision-making on rates. But there were multiple clues on Wednesday that this is changing.

First was the release of the Fed’s summary economic projections. These showed that Fed policymakers’ median expectation is now for inflation, as measured by the core personal-consumption expenditures index, to come in at 2.5% next year, up from September’s projection of 2.2%. And the projections now imply just two quarter-point rate cuts in 2025, down from four previously. A greater number of policymakers also indicated higher uncertainty on core PCE.

“It’s hard to justify trimming the number of cuts in 2025…unless Fed officials are explicitly accounting for upside surprises in inflation from the incoming administration’s policies," wrote Omair Sharif of forecasting firm Inflation Insights in a note. “This looks to me like they are penciling in tariffs for next year."

Besides tariffs, tax cuts and curbs on immigration are likely on the agenda once Trump takes office. The degree and timing of all three are very much in question, but there is almost certain to be some of each. They would also to some extent be at cross-purposes with each other, some being stimulative while others would tend to hold back growth. But all three would likely have the impact of pushing prices higher.

One can hardly blame Fed officials for having this in mind. Simply waiting until these policies are crystal clear and imminent, and blithely going on cutting rates in the meantime, would risk falling behind the curve and allowing inflation to once again spurt out of control.

During his press conference, Powell was asked to what degree the Fed is expecting “inflationary fiscal policy next year." In a cautiously worded response, he acknowledged that “some people did take a very preliminary step and start to incorporate, you know, highly conditional estimates of economic effects of policies."

Rather than emphasize that these policies would be inflationary, however, he focused on the fact that there is now higher “uncertainty around inflation." Importantly though, he indicated that this uncertainty itself is a reason to move slower on rate cuts: “It’s kind of common-sense thinking that when the path is uncertain, you go a little bit slower. It’s not unlike driving on a foggy night or walking into a dark room of furniture. You just slow down."

Trump may not be fond of that metaphor to describe the coming start of his presidency, but it is apt. The degree of uncertainty around what exactly he will do upon taking office is very high, especially on tariffs. On Wednesday that reality began to sink in with investors.

Write to Aaron Back at aaron.back@wsj.com