Formally:

(Premise 1) tampering did occur
(Premise 2) tampering determined election outcome
------
(Conclusion ) election was stolen

Is invalid.

P1 True
P2 True
------
C False

Such scenario is possible.

Election wasn't stolen, then it must be the case that ether tampering did not occur or tampering did not determine election outcome.

For example, there was nothing but tamperng from both sides, as there was no legitimate votes casted, not even a single one. In this case P1 and P2 are true, but C is false. As such, your argument is (formally) invalid.

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