Here are some of the arguments you can expect the lawyers to debate in the Senate trial. The GOP's argument has been no laws had been broken so the president did not commit high crimes per impeachment criteria. The President did indeed break laws to execute his scheme. The government General Accountability Office just reported that Trump had broken the impoundment law by failing to provide a reason or following the required procedure in freezing the aid to Ukraine. https://www.gao.gov/assets/710/703909.pdf Another is that Federal Election Laws that forbid soliciting something of value from a foreign government in a political campaign. Trump broke that one by asking Ukraine to find dirt on a political opponent to help him in the 2020 election. That "favor" was asked of the President of Ukraine in the infamous July "perfect" phone call. Nothing in the Constitution requires the president of being found guilty of a crime before he could be impeached. If there was such a Constitutional requirement of the commitment of a crime prior to impeachment, there would never be an impeachment because no president can be indicted for a federal crime while in office due to court decisions and Department of Justice rules.
What we can expect is for the GOP to claim Trump's impeachment is baseless because Trump's attempt to play a dirty campaign scheme failed so there was no harm done. An attempted crime is considered a crime by itself even if it is stopped in the act. Timelines are damning evidence.
Expect the GOP to say that the President's actions were committed to helping American foreign policy objectives of ending corruption in Ukraine so therefore he did not abuse his power to help himself win re-election. The problem the GOP faces is that every witness and document revealed in the House inquiry only proves that Trump's single-minded goal was to get dirt on the Bidens by his constant failure to cite the need to end Ukraine's corruption in general. Even two participants in executing the scheme agree Ambassador Gordon Sondland concluded that Trump did not give a "s..t" about Ukraine; he was only interested in the Bidens. Lev Parnas, Guiliani's fixer and on the ground executor of the scheme, in an interview on TV testified likewise. If he is considered a flawed witness, Parnas mostly corroborated other sworn testimony and backed up his statements with documented text messages and notes. That is also a good reason he needs to be called to give sworn testimony in the trial. Both men also attested the President's private attorney, Rudy Guiliani, was not a rogue actor but that the president himself directed their activities. John Bolton, a party to the scheme, may also provide corroborating testimony.
The GOP leaders want to dramatize witnesses if witnesses are ever permitted are ones that put Joe Biden in a bad light, though Joe Biden is not the object of the impeachment and was not a witness or participant in the events of withholding aid and pressure on the Ukrainian president. Biden's actions may be relevant in a campaign, but he is not relevant to the issue now before the Senate. An inconvenient timeline...Ukraine corrupt prosecutor was not investigating Burisma, the company Hunter Biden was part of,. for corruption when Joe Biden was urging the prosecutor to be removed. https://www.nbcnews.com/politics/trump-impeachment-inquiry/biden-campaign-warns-against-media-use-trump-disinformation-during-impeachment-n1118681
The Government Accountability Office is a legislative branch government agency that provides auditing, evaluation, and investigative services for the United States Congress. It is the supreme audit institution of the federal government of the United States. Wikipedia
Update: January 15 and 17, 2020
Lev Parnas' document dump within the week is written evidence that additionally verifies Trump's intent in the actions was not on behalf of national policy but for his own political benefit. Trump is directly tied by those Parnas documents to directing the operation through Rudy Guiliani. Parnas and the testimony of John Bolton, Trump's national security advisor at the time, could provide further damning evidence of Trump's corrupt intent. Both indicated they would testify if subpoenaed in the Senate trial and present new evidence not available to the House inquiry.
We had made fun of Lev and Igor. No one is laughing now. Lev, born Russian, appeared both literate and accent free fluent in English and he was Guiliani's gopher. in Ukraine itself... He asked who was he himself, a nobody, and he said he would never have had access to the Ukrainian president and his brass if they did not believe he was representing Trump and was under his direction via Guiliani. His interview by Rachel Madow was most corroborating public utterance of what others had said during testimony, but he was the executor of the scheme itself on the ground there and the closest to the action. Like Fiona Hill, like Gordon Sondland, like so many others testifying at the impeachment inquiry, Trump was not interested in carrying out the US policy to reform Ukrainian corruption in general, but he was only interested in getting dirt on the Bidens. This is key since the GOP's main line of defense will be Trump was carrying out stated US foreign policy to end Ukraine's corruption. Amb. Sondland himself told those in earshot," Trump did not care "s..t" about Ukraine. It was about the Bidens." Finding dirt about the Bidens to help him in 2020 was the purpose of the scheme. The GOP will claim Lev is an untrustworthy witness because he is under indictment for campaign violations in 2016. However, whatever Lev said he had reams of text message printouts and damning notes he wrote on a hotel notepad to back his words up. His interview with Maddow was not under oath and I'll put money on it that Mitch McConnell will do all he can to keep him from being a witness under oath at the Senate trial.
The Parnas documents also expose another deal in the works. The Ukraine prosecutor, Yuriy Lutsenko, fingered by the US Ambassador Maria Jovanovitch as corrupt, said he had dirt on the Bidens he would provide if Jovanovich was removed. She was removed quickly in fear she was in danger. and Parnas revealed the threat that she was being followed. That explains how Jovanovitch got reassigned so abruptly without explanation..For Guiliani to get the dirt, the deal was to get Jovanovitch out of her post first. Both the Ukrainian government and the US Secretary of State announced they would open an investigation to check into the implied threat of Yovanovitch being followed and for what purpose. https://www.nytimes.com/2020/01/16/world/europe/ukraine-yovanovitch-investigation.html
__________________________________________________________________________________https://www.vox.com/policy-and-politics/2019/11/20/20974201/gordon-sondland-impeachment-hearing-testimony-biden-show-trump Ambassador Gordon Sondland’s Wednesday, Nov. 20, House testimony was full of revelations. One of them is that President Donald Trump never seemed to ask Ukraine for an actual investigation of Vice President Joe Biden or his son, Hunter.
There is a great discussion of the meaning of high crimes in Wikipedia and the various interpretations.