Not in this case because the word base here does not refer to a "standard" level of which ultraX would be the opposite, but rather refers to a group of people w

Not in this case because the word base here does not refer to a "standard" level of which ultraX would be the opposite, but rather refers to a group of people who support a particular political party. There's nothing oxymoronic about the idea that people with extreme views form the core support of a candidate or party.