[Reaganwuzthebest]

Candidates for the presidency have been given the equivalent of a presidential security briefing since the 1960s or 1970s.

Before giving a security clearance though wouldn't the government still do a thorough background check on the candidate? For all we know he could be a KGB-type mole. If these reports turn out to be true then that would mean there's been a serious lack of vetting on the part of whatever agency is responsible for doing the checks.

[Reily]

Nope
Political parties would go absolutely ballistic if that was required. (Well one political certainly would !)
If Candidate A was from the out of power political party, and candidate A didn't pass the security screening, Candidate A's party could claim that it was politically motivated.
Remember screening for clearances also might reveal felonies and embarrassing misdemeanors, politicians from either party would be uneasy about meeting those standards.