Thought experiment.

[FarCenter]

2. 1968 and 1980

In the end, Nixon’s October surprise trumped LBJ’s. On November 2, Thieu announced that “the government of South Vietnam deeply regrets not being able to participate in the [peace] talks,” and as quickly as it had emerged, the euphoria over LBJ’s October 31 announcement broke. Without South Vietnamese participation in the Paris talks, there was little chance of final resolution.

...

For many years, there were persistent whispers in the Beltway that Reagan's campaign manager, William Casey, who later became CIA director, colluded with persons close to the ayatollah to delay the release of the hostages. But there is no compelling evidence to suggest this ever happened, just as there is a scant basis for conservative claims that Senator Edward Kennedy used a go-between in 1984, in an unsuccessful attempt to involve the Soviet Union in the 1984 presidential election.

http://www.politico.com/magazine/story/2016/07/russia-dnc-hack-donald-trump-foreign-governments-hacking-vietnam-richard-nixon-214111

Since the US intervenes in foreign elections all the time, e.g. most recent Ukraine elections, it is natural that other countries would attempt to intervene in US elections.

In fact, sine the US is the "sole superpower", all countries have a national security interest in intervening in the US elections.