There are two articles of impeachment lodged against Trump. Both are very strong, but the second is even stronger than the first.
There is really only one plausible line of defense against the charge of impeachment based on abuse of power: it is to acknowledge that what Trump did was wrong, but that it doesn’t justify the momentous step of removing a president from office. It’s plausible because, if this really were the only abuse of power attributable to this president, I am confident that he would not have been impeached. Trump has been impeached not because the extortion of Ukraine was a unique abusive act but because it was a tipping point in a pattern of abuse that was documented by Mueller and evident in a range of other presidential misbehavior. As I have argued, the larger pattern should have been the subject of the impeachment, but the Democrats chose otherwise. So, Republicans willing to acknowledge Trump’s wrongdoing but still unwilling to convict would have a point. Of course, there are very few such Republicans. Most will insist, implausibly, that Trump did nothing really wrong.
On the other hand, there is no plausible defense whatsoever against the second article of impeachment—obstruction of Congress. The Constitution gives Congress the right to impeach and try a president. The president’s largely successful effort to prevent Congress from obtaining information—documents and testimony–needed to exercise that right is indefensible. The president, echoed by his supporters, justifies his stonewalling with the claim that the whole impeachment process is a hoax. But even if you believe that the charge of abuse is unfounded, you must acknowledge Congess’s right to investigate that charge. The constitutional provision for impeachment would be pointless and even absurd if the president had the right simply to reject any impeachment as illegitimate. Or if he had a right to claim executive privilege as pretext for impeding an impeachment process. (The Constitution provides for impeachment; it says nothing about executive privilege.)
So, Republicans, you can make the plausible claim that Trump’s Ukraine shakedown isn’t sufficient grounds for impeachment. But there is no way you can defend Trump’s denial of Congress’s constitutional right to impeach. That is why if you really care about defending the Constitution, you must vote to convict and remove the president for obstruction of Congress.