POTUSes have used withholding of aid/funds for decades to leverage foreign powers ingoing along with them, as well as their dispensing funds/aid for bribery. FDR tried to leverage Japan with an oil embargo & used "lend/lease to bribe Britain & to some extent, Russia. No one complained.
The difference here is that Trump mentioned Bidens name and he is a potential political rival. The other difference here is the Dems have wanted him impeached since before he took office. ITL is correct that there is no evidence that Trump withheld aid to get an investigation into Biden.
1. Biden was a legitimate concern with regards to corruption in the Ukraine. Trump did not just pull his name out of a hat. He has been effectively dodging questions on this but if he gets the nomination he will have to answer this.
2. The talking points of his phone call were brought to him by his national security advisers. They in fact had to talk him into the phone call.
3. The Ukrainian Prez said himself on national TV and in front of everyone that he was not pressured and there was no quid quo pro. When the Dems talk about all of this they conveniently ignore this.
4. The whistle blower is an Obama hold over and a political activist, his ties will be made clear before this is all over.
5. The Ukrainian Prez did not even know aid was being withheld. Another inconvenient fact the dems ignore.