I can only answer in basic terms, but since Manifest Destiny was a potent idea in the minds of many great American men. I say the Mexican American War and Polk’s annexation of the Southwestern territories was a big step towards this, giving the U.S. ample coastland for both the Atlantic and Pacific oceans.

Any steps towards becoming a world power were slowed to a crawl by the Civil War. The preservation of the Union and the failure of the United States to splinter helped shore up the reputation but they were still more so looked at as a regional power.

When the United States showed itself willing and able to influence the outcome of WW1 in Europe it began it’s entry into the world’s power politics. With Europe ravaged and the population growth suppressed, the U.S. was clearly the power least effected and resulted in a net positive their reputation. Another delay would occur as all those same countries grappled with the great depression.

The impact WW2 had in shaping the world, and the ascendency of the United States can’t be understated. Churchill needed the U.S. to enter the war and Roosevelt wanted the liberalization of global markets. This policy would weaken all the European powers and give the U.S. a crack at all the open markets. With Europe and Asia in ruins after the war and American production not just untouched but at remarkable levels, these newly free markets gave the U.S. and all it’s goods and food a place to go.

What was the public’s opinion on the superpower status? At the time there was no formulated opinion. Everyone just reacted to the new reality and the beginning of globalization. However their feelings on the way were documented. And in many ways it gives us a view into the American’s opinion on themselves. Read through some old Gallup polls: https://exhibitions.ushmm.org/americans-and-the-holocaust/us-public-opinion-world-war-II-1939-1941