During Election years, USSS borrows Special Agents from other agencies to assist with USSS protection duties. Pre-DHS it was other Treasury Department LE Agencies including US Customs and ATF.
USSS was rolled over from Treasury to DHS after 9/11 and US Customs Investigations was rolled into HSI/ICE when DHS was created.
Even with the help, being a USSS SA during an election year is a tough gig.