A member will veto an election:

If the member seeking an election is not a member of the voter’s set.
If the current primary has more recent operations (i.e. a higher optime) than the member seeking election, from the perspective of another voting member.
If the current primary has the same or more recent operations (i.e. a higher or equal optime) than the member seeking election.

bool TopologyCoordinatorImpl::_isOpTimeCloseEnoughToLatestToElect(
    const OpTime& otherOpTime, const OpTime& ourLastOpApplied) const {
    const OpTime latestKnownOpTime = _latestKnownOpTime(ourLastOpApplied);
    // Use addition instead of subtraction to avoid overflow.
    return otherOpTime.getSecs() + 10 >= (latestKnownOpTime.getSecs());
}