\begin{table}[htbp]\centering
\def\sym#1{\ifmmode^{#1}\else\(^{#1}\)\fi}
\caption{Validation of Electoral Security Measures}
\begin{tabular}{l*{4}{c}}
\hline\hline
                    &\multicolumn{1}{c}{(1)}&\multicolumn{1}{c}{(2)}&\multicolumn{1}{c}{(3)}&\multicolumn{1}{c}{(4)}\\
                    &\multicolumn{1}{c}{Inc. Vote Share}&\multicolumn{1}{c}{Inc. Vote Share}&\multicolumn{1}{c}{Inc. Vote Share}&\multicolumn{1}{c}{Inc. Vote Share}\\
\hline
Electoral Security (MRP)&      0.0683\sym{***}&      0.0632\sym{***}&                     &                     \\
                    &   (0.00397)         &   (0.00408)         &                     &                     \\
[1em]
Electoral Security (Cook)&                     &                     &      0.0823\sym{***}&      0.0771\sym{***}\\
                    &                     &                     &   (0.00311)         &   (0.00336)         \\
[1em]
DW-Nominate (First Dim.)&                     &     -0.0611\sym{*}  &                     &     -0.0437         \\
                    &                     &    (0.0279)         &                     &    (0.0266)         \\
[1em]
House Tenure        &                     &     0.00411\sym{***}&                     &     0.00280\sym{***}\\
                    &                     &  (0.000742)         &                     &  (0.000686)         \\
[1em]
Majority            &                     &      0.0477\sym{+}  &                     &      0.0276         \\
                    &                     &    (0.0245)         &                     &    (0.0236)         \\
\hline
\(R^{2}\)           &       0.193         &       0.242         &       0.275         &       0.312         \\
Observations        &\num[group-separator={,}]{2726}         &\num[group-separator={,}]{2721}         &\num[group-separator={,}]{2726}         &\num[group-separator={,}]{2721}         \\
Number of House Members&\num[group-separator={,}]{544}         &\num[group-separator={,}]{544}         &\num[group-separator={,}]{544}         &\num[group-separator={,}]{544}         \\
Congress Fixed Effects&          No         &         Yes         &          No         &         Yes         \\
\hline\hline
\multicolumn{5}{l}{\footnotesize Heteroskedasticity robust standard errors clustered at the member level in parentheses}\\
\multicolumn{5}{l}{\footnotesize \sym{+} \(p<0.1\), \sym{*} \(p<0.05\), \sym{**} \(p<0.01\), \sym{***} \(p<0.001\)}\\
\end{tabular}
\end{table}
