Donald Trump is not going to win. To understand this you need to understand that the United States is very divided into two competing parties.
Our conservatives (be careful, the US definitions of liberal and conservative don't strictly match European definitions) are represented by the Republican party. This includes people who support gun rights, conservative social values (think anti-abortion and anti-gay rights), and oppose immigration.
Our liberals are represented by the Republican party. This includes people who support civil rights (i.e. same sex marriage and protection for minorities), immigration and social programs.
Of course there are people in the middle... but Americans right now are pretty divided along these lines. For example there are very few people are are in favor of both gun rights and same sex marriage (although there is no reason for this).
In the past couple of decades, the American people have moved toward the Democrats, becoming more sensitive to minorities and even favoring same sex marriage. The Republicans have responded by going complete nuts. The most radical among them have become angrier and angrier. This has made more people in the middle move toward the Democrats which has accelerated this vicious cycle.
Now to the primaries (the part of the election that is happening right now).
We first have each party (the Democrats and the Republicans) vote separately to decide which person will represent each side in the grand final showdown.
Trump is winning for the Republicans... he is winning by being angry and loud. Being angry and loud is a very good way to get the votes of these Republicans who are already feeling the country move away from them. They started out angry, and Trump is doing a very good job feeding into this.
Of course, the country as a whole isn't like the Republican party. As a whole most of us are happy about same sex marriage and don't hate immigrants or think guns should be unregulated.
So don't worry, the Republicans are going crazy. We are basically witnessing the break down of a party that used to represent a broad right of center coalition but now is the party of angry crazy people.
Getting a majority of angry crazy people to vote for him is within Trump's grasp.
However, when Trump has to face the nation as a whole, he won't find there are enough angry crazy gun-nuts to vote him into office.
So yes, I am taking him seriously. I am sure as hell going to vote... and I am going to do all I can to get sane people I know in swing states (another American political oddity) to vote too. But honestly, I am not too worried.