Verizon Wireless has been a huge backer of the Android operating system. The operator has thrown its formidable marketing muscle behind Motorola’s Droid franchise and currently carries more than a dozen Android devices, with more on the way.

Now that Verizon will be offering the iPhone, those marketing dollars no doubt will be divided between Android and iOS. Some believe the attention and marketing diverted to the iPhone could be detrimental to Android’s strength at one of its largest operator promoters. As NPD Group director Ross Rubin puts it: “The majority of smartphone sales at Verizon have been Android devices. That would be the operating system with the most to lose there.”

Verizon isn’t going to let its considerable investment in Android go to waste, but the iPhone likely will take attention and advertising funds away from devices using Google’s rival operating system. Together with Motorola and Google, Verizon is estimated to have spent $100 million marketing the Droid.

ABI Research analyst Kevin Burden says Verizon likely will shift its marketing away from a heavy focus on Android toward highlighting the variety of its offerings. “They’ll continue to be ground zero for Android, but now they have the iPhone. Its marketing will be more about the choice and flexibility consumers have with Verizon’s network,” Burden says. “I think they’re going to balance it and say it’s about choice.”

Burden also suspects that the iPhone could limit the number of Android devices Verizon decides to bring on board in the future. “Verizon might look at some Android phones now and say, ‘We don’t really need them,'” Burden says. “It’s going to say no to some Android devices – it has to manage the load balance on its network.”

Of course, Verizon isn’t the only operator to be heavily invested in the Android operating system. Sprint’s Android lineup includes the WiMAX-capable HTC Evo and Samsung Epic, T-Mobile USA currently lists more than 10 Android smartphones and U.S. Cellular sells five Android devices. Even smaller operators like Cellular South and MetroPCS are getting in on the Android game.

Android had a good run with Verizon, but now must share the limelight with the iPhone. It’s unclear how many Verizon customers will pick the iPhone over Android, but one thing is certain: Verizon’s Android devices are facing some steep competition.