Yes, that might be the reason. Google indexes & analyzes web pages and attempts to deliver contextually relevant ads. So, pages where "Trump" is mentioned may be more likely to display ads that are related to Trump. That's only one of the ways which Google targets ads, though.
It might even be that you were once on the website that is running the Trump ad and now they are retargeting ads to you. With retargeting, Google knows that you were once on the advertiser's website and displays their ad to you (if that is how the advertiser wants to target their ads, to people who have already visited their website). Google would know you were on that website only if the advertiser placed a special Google code on their site in order to enable for this type of targeting.
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