The American economy is showing signs of cooling, and Wall Street is counting on Jerome H. Powell, the chairman of the Federal Reserve, to come to the rescue.

The big question facing the Fed is when, not whether, to act, as far as investors are concerned.

But the Fed faces a tricky balancing act as it tries to parse economic data that are routinely complicated by President Trump’s unpredictable policies.

Mr. Powell signaled last week that policymakers were ready to help prop up the economy should mounting risks from Mr. Trump’s continuing trade battles and slowing global growth hit the United States economy. The jobs report last Friday showed that gains slowed sharply in May, prompting Fed watchers to expect rate cuts sooner rather than later.

Just last month, Fed officials agreed to leave rates unchanged as Washington’s trade war with China seemed on the brink of being resolved. But Mr. Trump renewed his fight just days after that April 30 and May 1 meeting, raising tariffs on $250 billion of Chinese goods and threatening to tax nearly every Chinese import. China has since retaliated with higher tariffs on American products.