NEW YORK—Donald Trump offered an expanded economic blueprint and outlined an overhaul of his tax plan on Thursday, but skeptics in both parties questioned his promise to offset steep tax cuts with significantly stronger economic growth.

In a speech at the Economic Club of New York, the Republican presidential nominee sought to cast his vision for the economy as the only one focused on boosting growth, framing Democratic nominee Hillary Clinton’s policies as based instead on redistributing the gains of a slower-growing economy.

...