"Tantamount to election" is a phrase in the United States to describe a situation in which one political party so dominates the demographics of a voting district, that the person winning the party nomination for a race (whether by primary or another method) will virtually be assured of winning the general election. The phrase "safe seat" is commonly used to describe such a district, though "safe" refers only to the general election, especially after the breakthrough of primary elections on nearly all levels.
The phrase originated in the American Solid South when and where 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. A counterexample to this is Vermont, where no Democrat served as governor between 1854 and 1963.
The phrase "tantamount to election" may, nonetheless, be employed to describe an electoral situation in an overwhelmingly Republican area where candidates of the Democratic Party (or vice versa) are up against very steep odds. It can refer to any electoral constituency in which dominance by one party renders candidates of other parties irrelevant.