It's about as straightforward as a thing can be: the President of the United States has used his powers as commander-in-chief of the armed forces and head of state to coerce another country -- one which relies on us for protection against a stronger, threatening neighbor -- to dig up dirt on his domestic political opponents. That's the textbook definition of abuse of power. One could not invent an example of an abuse of power more on-the-nose than this.
If Congress does not impeach the president for such an act, he is completely and utterly above the law.
It does not matter that the Senate will almost certainly not remove him from office. Doing noting establishes the most dangerous of precedents. Not only with respect to this particular abuse of power but with respect to the fundamental ability of Congress to exercise oversight of a president. If Congress does not impeach here, it is empowering future presidents to abuse their power in similarly brazen and destructive ways.
Force the president to at least to begin to answer for his illegal acts. Force his political allies to stop their ridiculous equivocating and vote, on the record, in support of this. History will look terribly upon this president regardless, but it will look even more terribly upon those who stood by, watched what he did, knew that he abused the power of his office, and let it pass without making even the slightest effort to hold him to account.