Complex sentence: Kennedy vetoed McGovern's suggested choice of Boston Mayor Kevin White and the Massachusetts delegates threatened to boycott the convention hall if the choice was White, who--as an Edmund Muskie supporter--had fought sharply with the McGovern slate during the primary.
Split into simpler sentences: Kennedy vetoed McGovern's suggested choice of Boston Mayor Kevin White. And the Massachusetts delegates threatened to boycott the convention hall if the choice was White, who as an Edmund Muskie supporter had fought sharply with the McGovern slate during the primary.