Trump is already walking back his most important promise on the economy

President Donald Trump rose to power in a surprise electoral victory after promising a disaffected electorate that he could more than double the US's rate of economic growth to 4%.

Barely a month into Trump's presidency, his advisers have already downgraded that rather elevated forecast.

Steve Mnuchin, Trump's recently confirmed Treasury secretary who is a former banker at Goldman Sachs, has been making the rounds talking up plans for a round of tax cuts. At the same time, he has substantially downgraded the White House's goal for economic growth — to just 3%.

The White House website still boasts that "to get the economy back on track, President Trump has outlined a bold plan to create 25 million new American jobs in the next decade and return to 4% annual economic growth."

But in an interview with CNBC, Mnuchin was signing a much softer tune. "We believe we can get back to sustainable growth of 3% or more," he said.

To put the difference in perspective, a full percentage point of growth could equate to millions of new jobs, so the margin of error is hardly trivial.

Investors might be reminded of a recent pattern at the Federal Reserve, which has consistently overestimated the likely strength of US gross-domestic-product growth at the start of each of the past several years only to turn tail as conditions worsened or unexpected shocks hit the economy.

It already seems to be doing the same this year, having pegged itself to the prospect of three interest-rate increases that look increasingly unlikely.

This pattern has damaged the Fed's credibility with financial markets and the broader public — a trend that could haunt the Trump administration as its campaign hopes inevitably collide with the realities of government.

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