Tuesday is either report card day or judgement day for Apple, depending on your particular doom-and-gloom scenario. The iPhone maker is expected to report a flat mid-summer quarter as investors question whether the best days of the company are behind it.

At the same time, observers see the third quarter as peculiar, given that no new iPhone or iPad has been announced, opening up CEO Tim Cook’s leadership to speculation and questions whether the iPhone maker can come back from two consecutive quarters of disappointment…

In the usual lead-up to quarterly financials, there is a consensus of $35.09 billion for the three-month period, below Apple’s own expectation of between $33.5 billion to $35.5 billion. According to Fortune’s survey of analysts, independent expert’s are a bit more optimistic, forecasting $36.15 billion.

Mac units sales by quarter (201306, Fortune 001)
Mac unit sales by quarter. Chart courtesy of Fortune.

Among the independent analysts, Ilari Scheinin of the Braeburn Group is predicting a blowout with $38.3 billion while Rajesh Agarwal of Braeburn expects $34.11 billion. Among institutional investors, Daniel Ernst of Hudson Square is most optimistic, suggesting $36.23 billion, with Christopher Caso of Susquehanna forecasting $32.84 billion.

iPad units sales by quarter (201306, Fortune 001)
iPad unit sales by quarter. Chart courtesy of Fortune.

As Fortune’s Philip Elmer-DeWitt notes, the likemindedness we saw with independent and institutional Apple investors during the second quarter has vanished. At the heart of the confusion and a recent spate of trimmed estimates are iPhone sales.

iPhone units sales by quarter (201306, Fortune 001)
iPhone unit sales by quarter. Chart courtesy of Fortune.

Will greater demand of low-cost handsets in places such as China and India balance a slowdown in sales of high-priced smartphones, such as the iPhone 5? The iPhone’s growth of 113 percent in 2011 or the 28 percent growth of 2012 is expected to drop to just 4.3 percent during the third quarter, according to Fortune.

Another factor in the uncertainty: Verizon Wireless recently announced it may have to pay Apple billions for unsold iPhones. The iPhone, released during rising interest in smartphones, must now deal with maturing markets where essentially anyone who wants the more powerful phones already has one.

To increase sales, U.S. carriers have unveiled differing plans all with the same objective: get smartphone owners to upgrade sooner.

Verizon Wireless store (interior 001)

In the past, Apple has been able to goose sales by announcing a new product or service, usually between April and June. However, this will be the first such period since 2009 that Apple hasn’t done so, notes Erica Ogg of GigaOM.

Apple is betting it can pull off another take-no-prisoners holiday period, unveiling a likely iPhone 5S and a new iPad – maybe an Apple TV variant with lots of network goodies – just in time for the all-important holiday shopping season.

How will the third-quarter turn out?

Check back later Tuesday, when Apple reveals the numbers – good, bad or just ugly.