Complex sentence: Nonetheless, the negative backlash over the television spot made the Tories' defeat certain as their support plummeted into the teens, all but assuring that the Liberals would win a majority government.
Split into simpler sentences: Nonetheless, the negative backlash over the television spot proved to be the final nail in the Tories' coffin. Their support plummeted into the teens, all but assuring that the Liberals would win a majority government.