I can't imagine anything more deserving and worthy of impeachment than withholding Congressionally approved money to exhort another country to dig-up bogus dirt on a political opponent. That is simply indefensible. Every Senator who votes to acquit Trump should have to say if they think it's now okay to ask a foreign country to interfere in elections. Or is it only okay if a Republican does it?
Is Trump scared of Biden? I have no idea, and I don't care, that doesn't change the fact that what Trump did was corrupt. There is no guarantee that Biden will even ultimately be his opponent, which makes it extra stupid of Trump to have risked his presidency on that. Although it is also not relevant here, for the record I am not a Biden supporter.
Republican Congressmen and Senators also know that Biden getting a prosecutor fired in the Ukraine in-line with official government policy, and Congress being made aware of it, is not equivalent to Trump's secret maneuvering for personal benefit. It is disingenuous to pretend they are equivalent.