Did Trump cause mainstream media to become overtly leftist? Not exactly.

Mainstream media’s general bias against President Trump has been obvious since he secured the GOP nomination for president in 2016. Prior to that, he was a media darling. It’s as if they wanted him to win the nomination thinking he would easily lose to Hillary Clinton, so they showered him with neutral or even positive coverage until no Republican could beat him. Then, they turned on him.
Obviously, it didn’t work, and since the election essentially every major media outlet other than Fox News and the Wall Street Journal have been anti-Trump.
Did he cause their shift from at least pretending to be impartial to the current display of outright unabashed bias? No. Donald Trump’s election as President has simply been the cover the media needed to do what they’ve wanted to do for decades: embrace a leftist worldview and participate directly in its ascension in the hearts and minds of the masses.
If anything, mainstream media owes the President their gratitude. He gave them the cover they needed to shed their false neutrality and walk they way they’ve wanted to walk for a long time. The media hasn’t always been leftist; one can look at the state of media coverage five or six decades ago and see that it was relatively neutral and often leaned a bit to the right. But over the last three or four decades it has shifted away from the populist conservatism of newsmen of old and followed the Washington Post, NY Times, and CNN down the road of pure leftism.
Today, there’s no need for subterfuge. They don’t need to be unbiased because the masses no longer expect it. They don’t send letters or telegrams to CBS headquarters over reports like Dan Rather’s Killian story; today, it’s just assumed journalists will attack Republicans whether there’s merit or not. They’ll jump on any story that paints a Republican in a negative light while reluctantly reporting anything that works against Democrats.
Is mainstream media simply reacting badly to Trump’s presidency? No. They used it as the excuse they needed to flamboyantly reveal their true colors. This isn’t a shift the President forced. This is the shift the media has wanted for a while.