Why was President Trump congratulating Putin on winning the election heavily criticized?

[bytebuster]

Продовжуємо ганяти вату на Стек Ексченджі. Помітно, що раніше мої закиди про невизнаність кордонів моцкви і про нелегітимність Прєжнєва «не заходили» у маси, а зараз — заходять.

Q: Why was President Trump congratulating Putin on winning the election heavily criticized?

According to The Guardian, President Trump was subject to criticism over his decision to congratulate President Putin on winning Russian elections in 2018:

Donald Trump defended himself against criticism over his congratulatory phone call to Vladimir Putin following the Russian president’s recent re-election, insisting on Wednesday afternoon it was in US interests to maintain a positive rapport with Moscow.

AFAIK, congratulating a President for winning the elections is typically an automatic diplomatic gesture that is performed throughout the world and it happened before even for US–Russia (source):

• "former president Barack Obama also wished Putin well after the Russian election on March 4, 2012"
• "Former president George W. Bush also called Dmitry Medvedev in 2008"

Another example is a Romanian President who did a similar gesture, and Romania–Russia relations are not exactly full of unicorns and rainbows (also this source).

Question: Why was President Trump congratulating Putin on winning the election heavily criticized?

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A:

Because the very elections are disputed, and congratulating Putin on winning these elections is a semi-official recognition of these elections.

In 2014, Russia has committed an armed invasion, military occupation, and subsequent attempted annexation of Crimea.
The vast majority of the world countries neither accept nor acknowledge the Russian sovereignty over Crimea. The US repeatedly condemn the occupation, too.
The 2018 elections were held in Crimea.
Hence, the legitimacy of Putin's presidency is disputed.
The White House national security team reportedly warned Trump, literally written in capital letters, "DO NOT CONGRATULATE".

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Therefore, the criticism is based on the following reasons:

1. The legitimacy of Putin as a president is disputed. Any kind of diplomatic action should be carefully considered before the national security team can elaborate an adequate response.
2. Trump had gone against the advice of his own team, which indicates a deep schism even among the top WH officers.
3. This also makes stronger the argument of those who accuse Trump of The Collusion, and the accusations are now harder to deny;
4. The arguments about 2012 do not apply, as shown in existing answers to this question.