The principle that foreign aid can be used as leverage to get another nation to investigate something that the US wants investigated is not something that Democrats would deny in other circumstances. Nor is the possibility that foreign corruption would implicate a Republican leader something that would cause Democrats to hold back from pressing for an investigation. They would insist that politics really had nothing to do with it — they were just honestly pursuing truth in an important matter reflecting on the integrity of, say, US elections. Naturally, with the roles reversed, Republicans would be making many of the arguments we now hear from Democrats. But the idea that suspected corruption should be overlooked because it might have partisan overtones is absurd: one of the very reasons we have a competitive party system is so that each party will check the abuses of the other. If the parties were to agree informally that no awkward questions would be asked about Biden business deals with foreign oligarchs so long as no such questions were asked about Bush or Trump business deals with similarly sketchy figures, that bargain would rightly be called ‘collusion’.
Democrats naturally don’t want to think the worst of the Biden clan — and they insist on thinking the worst of Trump. But even as embittered a figure as John Bolton appears to report that President Trump was concerned about much more than the 2020 election. The collateral damage Joe Biden’s presidential campaign might suffer from Ukraine’s investigations (and if he’s so totally above-board, why would he suffer any damage at all?) would be the price to be paid for guaranteeing that there was no undue influence in the 2016 election. After all, this is the same line Democrats take about the Russia investigation — sure, it might turn out to be a dead end (as in fact, it has proved to be), and yeah, it might cripple a Republican administration for two years and help Democrats in the 2018 midterms, but that’s just the price that must be paid for taking foreign interference seriously.