Shortages in SS agents doesn't happen overnight. It takes many years of budget cuts and "doing more with less" to lead to problems with understaffing.
There's also increased demand for SS agents, not just because its an election year and campaign season, but also because threats against SS protected candidates and officeholders are at record high levels. Social media, political extremism and radicalization, the conspiracy theory industry and unregulated proliferation of weapons of mass destruction among civilians is resulting in historic levels of violent threats to the public, elected officials and candidates.
Secret Service agents aren't made overnight. The unprecedented levels of violent, armed crazy people in the US wasn't something anticipated 10 years ago. JMO