No, you got that wrong.
US Marshals back up federal courts. They have the guns and the authority to lock people up including federal government employees and federal contractors.
If Obama orders his guns against the US Marshals, then there’s no question what happens next. But Obama is not that stupid.
I’m having trouble visualizing U. S. Marshals waltzing into the White House and putting the bracelets on Obama. Intimidation would work well first.