A survey by Robert W. Baird analyst William Power has found an extremely strong customer demand for the next-generation iPhone, which Apple (NASDAQ:AAPL) is rumored to be announcing in an event on September 12. Power surveyed 2,000 U.S. Internet users and found that 45 percent of those wireless subscribers who were currently eligible for a handset upgrade intended to buy an iPhone next. While 31 percent were undecided on their next device, only 22 percent said they planned to buy a Google (NASDAQ:GOOG) Android phone.

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Power concluded that the high demand could lead to unit sales of more than 50 million for the sixth-generation Apple smartphone in the U.S. alone, which would be “well above current forecasts.”

However, the analyst felt the high number of eligible upgrades could pressure margins for U.S. wireless carriers in the third and fourth quarters of the year. “AT&T (NYSE:T) appears to have the highest exposure among the iPhone carriers, with 42 percent of surveyed subscribers currently eligible,” Power wrote in a note on Thursday.

Users are hoping for the new iPhone to have a longer battery life, 4G LTE support, improved functionality for the Siri voice technology, a better back camera, and a larger screen, the analyst found.

Seventy-seven percent of current iPhone users plan to stay loyal to the brand, while 5 percent are planning to make the switch to Android. Among Android users, 48 percent plan to buy another Android phone next, while 17 percent are considering switching to an iPhone.

Samsung topped the list of preferred Android handsets and was followed, in that order, by HTC, Motorola, and LG. Of current Research In Motion (NASDAQ:RIMM) users, only 2 percent plan to buy another BlackBerry. Interestingly, while 51 percent of those surveyed said they would be interested in a fictional Amazon (NASDAQ:AMZN) phone, only 7 percent expressed any sort of interest in a possible Facebook (NASDAQ:FB) smartphone.