So you "feel" like he isn't a liberal even though his long history reveals him to be just that? lol! OK.
Plus implicit in South40's argument is the idea that "past determines future."
This isn't just Trumps past. As recently as 2013 (2 years ago) he praised gun grabber Michael Bloomberg as "Brilliant" for "putting his money where his mouth is" in trying to take guns away from law abiding citizens. He has also said the sham known as gay marriage "is the law of the land". He also recently said he would keep the good parts of Planned Parenthood. He also recently said he would allow the "good" ILLEGAL aliens to return to America. His liberalism isn't just in his past, it is also part of who he is now.
As for his past, Trump is not a politician so we do not have a voting record on which to judge. What we do have is his long history of supporting liberal causes and politicians. That should not and can not be ignored. It should be factored in with his current liberal leanings to determine if his is worthy of the Republican nomination. It's called vetting.
vet
verb
investigate (someone) thoroughly, especially in order to ensure that they are suitable for a job requiring secrecy, loyalty, or trustworthiness.