What does Washington mean by the phrase "the spirit of party"?

What Washington called "the spirit of party" was, he argued, "inseparable from our nature, having its root in the strongest passions of the human mind"-namely, the tribal passion to separate the world into "we" vs. "they," into rival and competing groups. This spirit can then easily become a desire to see "our side" win, regardless of whether our side is better than "their side" and regardless of the issues at stake or the facts of the matter.