\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. Won}&\multicolumn{1}{c}{Inc. Won}&\multicolumn{1}{c}{Inc. Won}&\multicolumn{1}{c}{Inc. Won}\\
\hline
Electoral Security (MRP)&      0.0299\sym{***}&      0.0193\sym{***}&                     &                     \\
                    &   (0.00441)         &   (0.00428)         &                     &                     \\
[1em]
Electoral Security (Cook)&                     &                     &      0.0369\sym{***}&      0.0252\sym{***}\\
                    &                     &                     &   (0.00451)         &   (0.00427)         \\
[1em]
DW-Nominate (First Dim.)&                     &      0.0267         &                     &      0.0331         \\
                    &                     &    (0.0319)         &                     &    (0.0321)         \\
[1em]
House Tenure        &                     &     0.00945\sym{***}&                     &     0.00899\sym{***}\\
                    &                     &   (0.00100)         &                     &  (0.000980)         \\
[1em]
Majority            &                     &     -0.0281         &                     &     -0.0347         \\
                    &                     &    (0.0295)         &                     &    (0.0299)         \\
\hline
\(R^{2}\)           &       0.018         &       0.089         &       0.027         &       0.094         \\
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}
