No. That is not a reasonable argument. It merely goes along with the (rather stupid, IMO) classification of "Soccer Moms" or "Angry White Males" or "Nascar Dads" and then claiming that the candidate won because of a particular group. To sum up: there are 2 fundamental flaws in this type of analysis.
Flaw #1 -- everyone belongs to multiple groups. After all, one can simultaneously be a member of "Soccer Mom", "Evangelical Christian", "Black Matriarch", and so forth. Classification is itself inherently flawed. Ultimately, we are all individuals.
Flaw #2 -- when multiple "groups" (or rather, a majority of the members of those groups) vote for a candidate, it is inherently flawed to pick _one_ of those groups and say that they were the case of the candidate winning.
I suggest the media does this for a number of reasons:
(1) To make a story, in a limited amount of space (print media) or time (broadcast media), they need to simplify.
(2) The media thinks the sheeple are stupid, so they simplify the story.
(3) The media as a whole is "against" Evangelical Christians, so this rings a "hot button" for the typical media person (of course, there I go "classifying" so this number 3 is flawed in an of itself, yes?) But in fact -- if you look at the major media, other than major events like the death of the pope, or in the ghetto of the "Sunday religion page" in the paper -- the majority of stories involving Christianity are negative. They simply do not consider other types of events involving Christians or Christianity to be "news", so it isn't covered. Go and read for yourself. If there is a story in which religion as religion is part of the story -- it will almost always be negative in some way, or be involved in some form of contraversy. Other material is simply never covered -- from the perspective of the media, it simply isn't "news".
I will give a personal example. My church gives 25% of all income from tithing to "missionary work". It has done this year in and year out for many years. Most people think "missionary work" is something from the 19th century, or is some kind of patriarchical cultural imperialism (note the loaded concepts there). But this is what much of missionary work involves:
(a) Build an orphanage.
(b) Build a medical clinic.
(c) Build a school.
(d) Provide clothing to the orphanage.
(e) Provide medical supplies to the clinic.
(f) Provide books to the school.
(g) Provide soup to "street kids" in Moscow - and other nasty environments.
(h) Provide air service to fly supplies into those clinics and schools in the remote areas of the world.
(i) Care for those orphan kids -- feed, house, and educate them until they are grown. Because otherwise -- no one else will, and they will die of starvation or possible grow up as "street kids" sniffing glue and surviving as prostitutes.
(j) And yes -- translate the Bible into all those languages and provide teaching of the Word to those who want to receive it.
Now -- to some people this activity would qualify as "news". But you won't see this covered in the media. And if you haven't set foot into a church, you won't know this is going on. And it isn't just the above. My church specializes in this kind of work -- other churches do local support for the homeless, for abused children, for battered women's shelters. You won't see news coverage on this either. After all...it isn't "news"....