@spendius,
That these leads and lags, surgings and saggings are simply reported as facts and not explained suggests that talk about centralisation and regionalism is not polite in US chattering circles which are usually federalising.
They might be explained by how different states interpret the nuances of the candidate's pitch in the light of their own interests.
To what extent are members of Congress seen as representatives, ambassadors even, of the areas they are elected in or as legislators for a national entity. Such a balance exists in most countries which are coalitions of regional entities but it seems more pointed in the US than elsewhere of developed democracies.