"Tantamount to election" is a phrase describing the situation in which one political party dominates the demographics of a voting district to such a degree that the candidate winning the nomination of that party for a race (whether by primary or another method) will be virtually assured of winning the general election. The phrase "safe seat" (referring to the general election) is commonly used to describe such a district.
The phrase is most commonly used in the United States, often describing the Solid South where for decades the Republican Party was so weak or nonexistent that the general elections were mere formalities, the election having effectively been decided within the Democratic Party. For example, the state of Alabama, which was heavily Democratic, did not have a Republican governor or lieutenant governor between 1874 and 1987. Another example in the opposite political direction is Vermont, where no Democrat served as governor between 1854 and 1963.