Incumbent ideology, district ideology, and candidate entry in U.S. congressional elections, 1954–2008

• I model quality candidate entry in House and Senate elections from 1954 to 2008.
• The effect of incumbent ideology on entry is conditioned by district ideology.
• Ideological extremity helps incumbents avoid challenges in more extreme districts.
• In less extreme districts, extremity makes quality challenges more likely.
• Republicans gain more from ideology than Democrats in terms of challenger quality.

This paper investigates the role of incumbent ideology on quality candidate entry in U.S. congressional elections. Looking at district ideology, the analysis shows that members of the House and Senate face different ideological incentives in terms of opponent quality. For officials from highly ideological districts, greater ideological extremity is advantageous as it discourages strong opponents. For members from cross pressured districts, however, greater extremity may increase the likelihood of facing a strong opponent. Although the magnitude of these findings vary by party, they illustrate that incumbents face diverging ideological motivations in terms of discouraging strong challengers based on the ideology of their district.