URLs break compilation

Question

Just as a preamble, I use MikTex 2.8, the latest TeXworks for Windows, the natbib, hyperref, and pdftex packages, JabRef to manage my citations, and the pdfLatex option in TeXworks for compilation.

Everything works correctly and as expected -- unless I include the URL of an electronic paper I am citing. Then, compilation crashes, with the following error:

! Missing $ inserted.
<inserted text> 
                $
l.321 ....com/sol3/papers.cfm?abstract_id=1697352}

Here is the minimal example which generates this error:

\documentclass[12pt, letter]{article}
\usepackage[pdftex]{graphicx}
\usepackage{natbib}
\bibpunct{(}{)}{;}{a}{}{,}
\usepackage[pdftex,
        urlcolor=rltblue,       % \href{...}{...} external (URL)
        filecolor=rltgreen,     % \href{...} local file
        linkcolor=rltred,       % \ref{...} and \pageref{...}
        citecolor=black,
        colorlinks=true,
        hypertexnames=false,
        bookmarks=true,
        bookmarksnumbered=true,
        bookmarksopen=false]{hyperref}

\begin{document}
\pagestyle{plain}

Testing. \cite{Shor:2011all}

\bibliographystyle{apsr}
\bibliography{testurl}

\end{document}

The testurl.bib file, includes the item in question this way:

@ELECTRONIC{Shor:2011all,
  author = {Boris Shor},
  year = {2011},
  title = {All Together Now: Putting Congress, State Legislatures, and Individuals
    in a Common Ideological Space to Assess Representation at the Macro
    and Micro Levels},
  url = {http://papers.ssrn.com/sol3/papers.cfm?abstract_id=1697352},
  owner = {Boris},
  timestamp = {2010.04.04}
}

As you can see, it references the url of the electronic document (unpublished paper) I am trying to cite from SSRN: http://papers.ssrn.com/sol3/papers.cfm?abstract_id=1697352

The generated .bbl file contains the following item:

\harvarditem{Shor}{2011}{Shor:2011all}
Shor, Boris. 2011.
\newblock ``All Together Now: Putting Congress, State Legislatures, and
  Individuals in a Common Ideological Space to Assess Representation at the
  Macro and Micro Levels.''.
\newline\harvardurl{http://papers.ssrn.com/sol3/papers.cfm?abstract_id=1697352}

What's going wrong?

Answer 1

Since you use hyperref, you don't need to load the url package. After loading hyperref, you could redefine \harvardurl:

\renewcommand{\harvardurl}[1]{\textbf{URL:} \url{#1}}

Orignally, natbib defines it by

\newcommand\harvardurl[1]{\textbf{URL:} \textit{#1}}

This definition doesn't work with underscores in the argument. \url fixes it. If you would like to have italic urls, you could achieve that for example by

\renewcommand*{\UrlFont}{\itshape}