I know the media and Trump himself love to mention his lead in the polls, as if everyone forgot the roller-coaster ride of 2012. Here is the current RCP average for 2016:
As you can see, Trump peaked at 24.3% and now is at 22.8% and appears to be rolling over. Now let's look at the 2012 nomination race:
As you can see, there were lots of folks who had their turn to be in the lead in national polls but in the end fizzled out and all of them had higher peaks than Trump. Rick Perry peaked at 31.8% in September of 2011. Herman 9-9-9 Cain peaked at 26% in October. Then Newt peaked twice, once at 32.8% in December and again at 31.3% at the end of January (note that despite these peaks around the IA and NH primaries, he lost both). Finally, the lowly Rick Santorum had his turn in February with 34.2%.
Trump hardly has an insurmountable lead in the polls and very little of his support is actually committed to him.