If you ask the Democrats, one reason that Trump should be impeached is that he tried to extort Ukraine to investigate a long-debunked conspiracy about Joe Biden and his son. Put in more general terms, it could be said that one reason the Democrats want to impeach Trump is that he tried to push an investigation of Joe Biden and his son. Interestingly, some Republicans are accepting this claim and using it to defend Trump.
Taken without context, the claim “one reason the Democrats want to impeach Trump is that he tried to push an investigation of Joe Biden and his son” can be interpreted in many ways. Some of Trump’s defenders have made the claim that the Democrats are trying to impeach Trump out of revenge because of what he tried to do to Joe Biden. While this sort of revenge motive would certainly be morally problematic, their motivation is irrelevant as to whether Trump committed acts that warrant impeachment. To use an analogy, the fact that a prosecutor wants to lock up criminals because his family was hurt during a robbery does not entail that the criminal she is prosecuting is not guilty of the crime.
It is certainly fair to consider that motives can bias people, which is why the legal system (at least in the ideal) strives to have objective judges. One unfortunate aspect of almost any impeachment is that congress is made up of politicians who generally have party loyalty and thus it is almost always reasonable to see them as biased. Bias cuts both ways—those defending the president can generally be as biased as those favoring impeachment. While this does show a clear issue with impeachment, it does not count in general against the Democrats more than the Republicans.
A more interesting defense is that the Democrats are taking the view that Trump should have not pushed the investigation (into the debunked conspiracy theory) because they think that Biden, as a candidate, should be protected. As some of Trump’s defenders have noted, this would entail that candidates for office should be immune to investigation. The obvious point here is that we should infer that since this is absurd, the Democrats are wrong to try to impeach the president over this—after all, they also say, he was (incompetently) trying to root out corruption.
I certainly agree that candidates (and office holders) should not enjoy immunity from investigation. But this is not what the Democrats are claiming. Rather, their concern is that Trump used his office to try to coerce Ukraine into “investigating” a debunked conspiracy theory about Joe Biden in order to gain a political advantage in the 2020 election. To use an analogy, it would be as if an official was being investigated for using the police department to harass civil rights workers and her defenders claimed that critics said that civil rights workers should be immune to police investigation. This is, obviously, not being claimed at all. Their concern is with the misuse of the office, not with the legitimate operation of the police. Likewise, the Democrats legitimate concern is with the misuse of the office, not with the legitimate investigation of corruption.