Among a hundred voters, only a few whose initial preferences were uncertain will vote differently about a school issue if their precinct is located in a school rather that in a church - but a few percent could tip an election.
If additional context is needed:
Similarly to tip the balance/scales (please read more here):
to give someone or something enough of an advantage to influence a result or decision
You could translate tip the election as:
to give someone or something enough of an advantage to influence a result of the election.
For example, Martians wanted to tip the election in favor of Mr. Smith.
As noted by @Lambie, tip the election can be also translated as tip the balance or scales of the election.
Martians wanted to tip the election in favor of Mr. Smith.