President Obama did everything he could to extend Olive branches to the Republican party, and try and get them to work together with him. In the first year of his presidency, he spent months trying to work together with Republicans on health care reform despite the fact that he had a super majority and could have passed any bill he wanted. He changed multiple aspects of the ACA to try and soften it and hopefully get even a few Republicans to join him even though it made the bill worse, and it bothered his own party.
Again when trying to repair our broken economy(which fell apart under the previous Republican) President Obama reached out to Republicans at every turn trying to get their support for his stimulus package even though he didn't need it. He offered to extend tax cuts on Millionaires and Billionaires desperate to try and unite this country behind a bipartisan fix.
In each case, the Republican party told President Obama to go **** himself. They didn't care about bipartisanship, they didn't care about solving problems, they didn't want to work together because they were scared if they did it would make President Obama look like a great leader, and they'd never be able to beat him.
Instead, **** heads like Trump ran around the country trying to convince everyone that Obama was secretly a Muslim born in Kenya and that he had illegally stolen the presidency. They didn't just criticize his policies or offer their own, they riled up their racist base and used it's anger to take back Congress. Every time President Obama and Democrats tried to take the Moral high road the Republican party went lower, and how was the Democratic party rewarded for that? The American people elected the very piece of **** that had gone around spreading all those lies about Obama to undo all the good he was able to accomplish.
If you don't reward the good guys and punish the bad guys, you're going to end up with more bad guys. Ignorant Americans have demonstrated that they don't reward compromises, and people who take the moral high road. Trying to work together with the modern Republican party is like Charlie Brown trying to kick Lucy's football. At some point Charlie has to realize that if he really wants to kick that football, he should kick Lucy in the ****, take the football from her, and go find someone else to play with.
The breakdown in bipartisanship in this country started with Republicans. They keep getting worse and worse, and they get away with it because Fox News tells their base that they're the good guys, and the average American is too stupid to know the difference. The only way we get back to unity and bipartisanship is if the Republican party wakes up and realizes what they've done and apologizes for it, or we the Democratic party destroys them entirely. As Democrats, we can only control one of those solutions.