Of course Quid Pro Quo is not inherently illegal, most transactions of any kind are Quid Pro Quo. But Quid Pro Quo that is achieved via Bribery and/or Extortion IS illegal.
I think they are so used to easily deceiving their Fox-addled base that Republicans thought deceiving actual thinking citizens would be just as easy. They expected that they could just gloss over:
-Republicans had full control of the senate, house, and presidency for 2 full years and there was never a peep about the Biden's alleged "corruption". They had 2 full years to investigate it and did nothing. It wasn't until Biden became a legitimate threat to his reelection aspirations that Trump decided to launch his extortion scam involving Ukraine.
-Trump demanded a PUBLIC announcement of an investigation, which blows up any claim that this was not primarily for the benefit of his reelection campaign. The only reason to demand an announcement is because he wanted to influence the vote in his favor. This is a thing of value, violating 52 USC 30121 right there.
-His actions afterwards, such as ordering his lackeys not to cooperate with the investigation, show a clear consciousness of guilt. He knew what he was doing was wrong and has taken several steps to hide it.