You will get consistent spacing if you use \raggedright
(either manually, with every cell in your table, or through an automated >{\raggedright}
fashion thanks to array
). But if you wish to keep the spacing for SOME LABEL
fixed, set it in a box (either \mbox
or \makebox
). However, I almost feel you need to set a list (using description
, for example).

\documentclass{article}
\usepackage{array,enumitem}
\begin{document}
\noindent
\begin{tabular}{@{} p{\linewidth} @{}}
\textit{SOME LABEL}\quad Lorem ipsum dolor sit amet, consectetur adipiscing elit $x_1 \ldots x_n$. \\
\textit{SOME LABEL}\quad Nullam nec mi et neque pharetra sollicitudin.
\end{tabular}
\medskip
\noindent
\begin{tabular}{@{} p{\linewidth} @{}}
\mbox{\textit{SOME LABEL}}\quad Lorem ipsum dolor sit amet, consectetur adipiscing elit $x_1 \ldots x_n$. \\
\mbox{\textit{SOME LABEL}}\quad Nullam nec mi et neque pharetra sollicitudin.
\end{tabular}
\medskip
\noindent
\begin{tabular}{@{} p{\linewidth} @{}}
\raggedright
\textit{SOME LABEL}\quad Lorem ipsum dolor sit amet, consectetur adipiscing elit $x_1 \ldots x_n$. \\
\raggedright
\textit{SOME LABEL}\quad Nullam nec mi et neque pharetra sollicitudin.
\end{tabular}
\medskip
\noindent
\begin{tabular}{@{} >{\raggedright}p{\linewidth} @{}}
\textit{SOME LABEL}\quad Lorem ipsum dolor sit amet, consectetur adipiscing elit $x_1 \ldots x_n$. \\
\textit{SOME LABEL}\quad Nullam nec mi et neque pharetra sollicitudin.
\end{tabular}
\medskip
\begin{description}[font=\normalfont\itshape]
\item[SOME LABEL]
Lorem ipsum dolor sit amet, consectetur adipiscing elit $x_1 \ldots x_n$.
Lorem ipsum dolor sit amet, consectetur adipiscing elit $x_1 \ldots x_n$.
\item[SOME LABEL]
Nullam nec mi et neque pharetra sollicitudin.
Nullam nec mi et neque pharetra sollicitudin.
\end{description}
\end{document}