First, one can consider it is temporal (and we do not map temporal things according to the guidelinces), but sometimes these campaigns take 1 year or more. Therefore it exist for some time.
Also, these office are important for the map as many people query where is the political campaign office, therefore it has a geographical value to locate it on the map.
I agree that, in some countries, campaign offices last just long enough to be mapped. However, their fleeting nature makes it difficult for this tag to achieve significant usage at any given time.
Last year, I went through the United States retagging any office=political_party that appeared to be a campaign headquarters as office=political_campaign. Only one of these offices remains, for a New Hampshire governor’s reelection campaign. I’m not certain this office is still open or, if it is, that it’ll be around for much longer. There used to be more such offices, like this New York mayoral campaign headquarters.
office=politician is a better-documented tag than office=political_campaign, but it refers to something akin to a constituent service office, which these definitely are not. I’ve been avoiding office=politician for either kind of office because of the likelihood of confusion. For those unfamiliar with U.S. politics, it’s kind of like confusing OSM with Mapbox just because both are staffed, in part, by mappers.
Whichever tag you settle on, I suggest setting operator:wikidata to the Wikidata item representing the campaign, or at least the item representing the candidate. That way mappers can more easily distinguish similarly named candidates and look up whether the candidate is still running for office. If not, that’s a good sign the office is closed and needs a resurvey.