Since 1920, how many U.S. House races have taken place with only one major party on the general election ballot?

Every two-year election cycle, some Democrats or Republicans win U.S. House elections without major-party opposition. Over the past 100 years, 14.4% of regularly scheduled U.S. House general elections had only one major party candidate.
From 1920 to 2018, there were 2,434 U.S. House races without a Republican candidate in the general election compared to 707 races without a Democratic candidate. In 2018, 41 of the 435 U.S. House races lacked either a Democratic or Republican candidate in the general election. Three of the 41 races did not have a Democratic candidate on the ballot, and the remaining 38 did not have a Republican candidate running.
The election years that had the most races without major-party opposition were 1930 (99), 1998 (95), 1942 (89), 1958 (89), and 1934 (83). Conversely, the election years with the fewest races of that nature were 1996 (21), 2010 (29), 1992 (31), 1932 (35), and 2018 (41).
On average across the 50 election cycles from 1920 to 2018, about 62.8 U.S. House races had only one major party represented on the general election ballot. During that timeframe, Democrats averaged 14.1 U.S. House races per cycle compared to 48.7 races for Republicans. In the 10 election cycles spanning 2000 to 2018, the average dropped to 57.4 races. In the 40 election cycles spanning 1920 to 1998, the average rose to 64.2 races. Alaska, Delaware, Idaho, Montana, New Hampshire, and Wyoming were the only states that had a Democratic and Republican candidate on every U.S. House ballot from 1920 to 2018.
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