With the economy on the line, the Fed prepares to take its biggest gamble in years

The Federal Reserve this week is all but certain to cut interest rates despite unemployment being at historic lows, a highly unusual action that is shaping up to be the biggest gamble of Fed Chair Jerome H. Powell’s brief tenure as leader of the world’s most powerful economic institution.

Many economists say the Fed is acting prudently to prolong the economic expansion, which this month became the longest in U.S. history. Yet the support is far from unanimous.

Some economists, Fed officials and people on Main Street say the Fed’s...