A fair and impartial observer would say that the two invasions were nothing like each other but the Democrats have such a hatred of Bush that they can't help but blame him.
Intent matters. There was never any question of the US annexing all or part of Iraq. In contrast, the former USSR had annexed the Ukraine for decades and Putin seems to desire to annex the Crimea.
In contrast, Putin invaded the Crimea a couple of days after its government fled and a new government was formed. The invasion was done with troops who wore no insignia on their uniforms.
Bush gave plenty of warning before invading and had wide international support with the "Coalition of the Willing".
The invasion of Iraq capped a decade of active hostilities beginning with Iraq's invasion of Kuwait. Even during the long lull, there wee weekly exchanges of rocket fire as the US enforced the no-fly zone.
There are a few differences that should be kept in mind.
After Russia invaded the Crimea, President Obama was left looking powerless. With Syria and other incidents, he has shown that he has neither neither plans nor will to back up his threats. Republicans were quick to point this out. They did not, as many Democrats have claimed, blame Obama for the invasion. They simply point out his powerlessness (and laughed at it).The Democrats have now stuck back. It turns out that the Russian invasion was caused by President George W. Bush's invasion of Iraq. That was such a terrible international crime that it changed international norms allowing any nation to invade any nation on any pretense.