Why do some scholars think that campaigns do not matter much for presidential election outcomes? Why do other scholars and practitioners think that campaigns do matter?

Scholars argue that there are three main reasons that campaigns do not matter in elections.  One, that other factors have a greater influence than campaigns, such as if a candidate is an incumbent, party identification of the candidate, state of the economy, and finally, macro-trends like wars and presidential approval.  Second, the public does not care about politics so they don't pay attention to campaign functions like speeches, candidate gaffes, or advertising.  Third, in presidential elections, each side has about the same amount of experts and money which cancels out any advantage those resources might bring.
Other scholars argue that campaigns do matter, and they also have three main reasons for this.  One, that after controlling for incumbency, campaign spending does impact a candidates performance. Secondly, Gerber and Green argue that campaigns personalized outreach, such as, door-to-door or digital contact, will influence the person contacted to go and vote for the candidate. Finally, for non-presidential elections, the campaign resources vary enormously between candidates and the campaign with the most resources is favored to win the election.