The election proper is in November. Right now, we're still running through the incredibly drawn-out nominations process for both parties.
Trump's popularity ought to be seen in context: so far, he's "only" received 7.8 million votes in all states, which represents less than 40% of the total. But those are, mostly*, votes from within the Republican party only. So already, some of his success is down to exploiting a divided field of candidates against him. There are still two opponents left, so the opposing votes remain divided, and the problem is that one of these people, Ted Cruz, is in a sense even more divisive than Trump is.
By contrast, the Presidential election vote thresholds are about 60 million or so. It is, at the moment, an open question as to whether or not Trump has the ability to increase his absolute support to those levels, particularly when he is so good at alienating large groups of people (including, now, women, who make up half the electorate). Presidential Candidate Trump is approaching almost certain. President Trump, less so -- but his opponent in such an election had better not be complacent, for sure.
*Things are a bit messy because each state has different voting rules; I think most are restricted to Republican voters, though.