Complex sentence: The California primary was "winner-take-all," which was contrary to the delegate selection rules; while McGovern only won the California primary by a 5% electoral margin, he took all 273 of their delegates to the convention.
Split into simpler sentences: The California primary was "winner-take-all," which was contrary to the delegate selection rules. So even though McGovern only won the California primary by a 5% electoral margin, he won all 273 of their delegates to the convention.