Right or wrong, better or worse, I think that Putin expected Trump to be reelected in 2020. Europe and NATO have proven to be more united and more opposed to the invasion than Putin anticipated. Given Trump's opposition to NATO, his acrimonious relationship with most European leaders, his poor relationship with the Ukraine, and his good relationship with Putin himself I think Putin thought his chances of success would be better under a second Trump administration. Again, it may well be a major miscalculation on Putin's part but I believe that was behind his decision. I have seen others post the claim that in a meeting with Putin, Trump told him that "If you invade Ukraine, I will bomb Moscow."
I don't know if this is true but others posting on this website have said this happened.
It sounds like something Trump could have done.