How to set leftmargin when using the enumerate package?

I recommend the package enumitem. Here you can set leftmargin local.

 \documentclass{article}
 \usepackage{enumitem,showframe}

 \begin{document}
\begin{enumerate}[leftmargin=2cm]
\item foo
\item bar
\end{enumerate}

\begin{enumerate}[leftmargin=1cm]
\item foo
\item bar
\end{enumerate}

\begin{enumerate}
\item foo
\item bar
\end{enumerate}
\end{document}

If you must use enumerate, you can start the optional argument of the enumerated list with an \hspace to push the whole list farther to the right:

\documentclass[leqno]{article}

\usepackage{amsmath,enumerate}

\begin{document}

Some text before the list.
\begin{enumerate}[\hspace{18pt}(P1)]
\item Some text.
\item Some text that goes onto two lines, so you can see where the second line will be aligned.
\item An equation:
\begin{equation}
1+1=2.
\end{equation}
\end{enumerate}
Some text after the list.

\end{document}

I don't know of a better way to do this with enumerate.