A number of President Trump's recent appearances around the country have been described as "campaign rallies." (Police use tear gas..., Trump, on Long Island,..., Trump threatens shutdown...) Though, notably, the phrases "campaign-style rally" and "de facto campaign rally" often appear, too.
President Trump is already a formal candidate for 2020 (FEC filing), and has been since January 20, 2016. This is unusually early: Barack Obama filed on April 5, 2011 to run in 2012; George W. Bush filed on May 17, 2003 to run in 2004; Bill Clinton filed on April 15, 1995 to run in 1996 (source, because I couldn't parse all the FEC paperwork for those three). So it raises the question:
When a(ny) president does become a candidate who pays for the rallies? I'd have naively assumed that "the taxpayer" pays for things like Air Force 1 and secret service, but who pays for a venue and vendors?
(Just to be clear, this isn't really a Trump-specific thing, just the prevalence of his rallies had me wondering. And it seems an opportune time to ask, as he's basically maximized how long a presidential campaign can last.)