When did working with another country to interfere with democratic elections become not a threat?
It’s always been a “threat”, from the way you defined that. The more difficult question is if the current Russian controversy fits that definition.
The larger question is the definition of “interfere”. As the largest economic force in the world, foreign actors have for decades tried to influence the US elections. For example:
1996 United States campaign finance controversy - Wikipedia
While questions regarding the U.S. Democratic Party's fund-raising activities first arose over a Los Angeles Times…
In this 1996 matter, China was alleged to have provided large sums of money to various Clinton campaign funds. And, similar to this cycle’s Russian influence dustup, there were calls for a special prosecutor, and there were allegations that the FBI had been politicized so as to protect Mr. Clinton.
So, these sorts of things happen from time to time. They need to be monitored when they occur, but trying to compare flows of money and information to a an actual physical threat like a bombing is not productive.
The other problem is that if you water down the definition of “interfere” enough, then the US is guilty of it too.