It seems that \hphantom{} is ignored at the beginning of an item. The MWE below yields:
How to get "First" and "Second" aligned?.
I know that you have to use \hspace*{} instead of \hspace{} at the beginning of a line, but it does not seem as if \hphantom*{} or \phantom*{} exist.
Code:
\documentclass{article}
\usepackage{enumitem}
\begin{document}
\begin{enumerate}
\item {}\hphantom{x }First Item.
\item x Second Item.
\end{enumerate}
\end{document}

\itemTeX is in vertical mode. – touhami Feb 02 '18 at 08:48\leavevmode\hphantom{x }– egreg Feb 02 '18 at 09:18tabular. I am not sure if had I seen that other question, if I would have realized it was the same problem. touhami's comment explains the problem and you provided the\leavevmode\hphantom{x }solution . So, as long as your comment does not get deleted, this is fine. – Peter Grill Feb 02 '18 at 19:57