“The ICA does not permit deferrals for policy reasons.” – General Accountability Office, commenting on the Impoundment Control Act, Decision B-331564, 1/16/20
With those nine words, the General Accountability Office  rendered the entire Republican defense of Donald Trump factually inoperable.
For the most part, there are three parts of the Trumpists’ defense of the president withholding Congressionally appropriated aid to Ukraine: first, that it was not an impeachable act because Trump was genuinely concerned about corruption in Ukraine; second, that abuse of power (the charge in the first article of impeachment) is void because no law was broken; and third, obstruction of Congress (the second article) is void because of the legality in the actions covered by the first article.
These have been the pillars of Republicans’ defense of Tump’s actions. None of them survive contact with the GAO report on the matter.
To understand why, we have to look at the GAO’s statement regarding the Impoundment Control Act (previous link, emphasis added):
Faithful execution of the law does not permit the President to substitute his own policy priorities for those that Congress has enacted into law. OMB withheld funds for a policy reason, which is not permitted under the Impoundment Control Act (ICA).
The Constitution grants the President no unilateral authority to withhold funds from obligation. See B-135564, July 26, 1973. Instead, Congress has vested the President with strictly circumscribed authority to impound, or withhold, budget authority only in limited circumstances as expressly provided in the ICA.
The ICA does not permit deferrals for policy reasons.
In other words, “concern about corruption in Ukraine” — even if true (and the evidence makes clear it isn’t) — is not enough to make the hold on funds legal, because no policy reason is enough to make the hold on funds legal. The first item of the Republican defense of Trump does not legally absolve the Administration.
Once that becomes clear, logic inevitably detonates the second argument, but the GAO also spelled it out in simple terms: “We conclude that OMB violated the ICA when it withheld USAI funds for a policy reason.” In other words, the Administration broke the law. The actions covered in the first impeachment article were illegal.
Logic then removes the support for the third GOP defense. Even if one were to claim that obstructing Congress isn’t a problem without an underlying illegal act, there is an illegal act behind the obstruction.
The Trump administration violated the law and obstructed Congress’ investigation of said violation; both were reportedly done on Trump’s orders. They are thus impeachable offenses and reason enough for the Senate to convict Trump and to remove him from office.