Republicans have had trouble mounting an effective defense on Ukraine because they haven't put down their stakes on the most defensible ground.
Complaints about House Democrats' less-than-transparent impeachment process, though justified, were clearly perishable once Democrats adopted more regular proceedings. The contention that President Donald Trump's phone call to the Ukrainian president was "perfect" was never going to withstand scrutiny. The line that there was "no quid pro quo" has become steadily less plausible as more testimony has emerged suggesting that Trump withheld security aid to Ukraine in the hopes that Ukraine would announce an investigation into the 2016 election and the gas company Burisma and/or Joe and Hunter Biden.