Around 5,000 officers will be on the streets, with police coming from across the UK to ensure Mr Trump's visit passes safely.

But that protection comes at a cost -- about $38,000 per day per candidate. The cost of providing Secret Service protection to presidential candidates exceeds $200 million.

The Secret Service had the former F.B.I. director James B. Comey followed by law enforcement authorities in unmarked cars and street clothes and tracked the location of his cellphone the day after ...