The fact all of the living former Presidents, from both parties, have offered the opinion that Donald Trump is unqualified to hold the office of President provides validation of President Obama's comment calling "uniquely unqualified" and a threat to the people of the United States as well as the world community.
He criticised Trump over apparently avoiding tax which merely showed that he is not willing to give anything back to the country that gave him everything. In the American history, this year election campaign was so nasty and arises one question whether the morality of the people had fallen below expectation.
Anyway, being a president of a country, I believe he should be moderate in supporting his nominee, because has a president, he should stand for all, and not for a selected number of people. However, playing the game politics, anyone can join to influence the result of the game. It is allowed.
No, I do not believe it is proper for President Obama to influence voters and the election by calling Donald Trump "uniquely unqualified". The president should not be allowed to campaign for either candidate during an election. It is an unfair practice that should be stopped. Perhaps Obama should focus on being president and less on Hillary.