The problem is that he, and a very significant number of Republicans, don't see the left as an enemy. They see them as competitors for their elected office, and their only concern from a ‘partisan’ perspective is to make sure they do whatever they can to ensure that they are in the best position to get reelected. In that regard, they will never stick their neck out any further than they think is politically prudent. In short, they are unprincipled selfish political hacks. Many would be perfectly happy to hold the same seat with a D after their name - as long as they could hold on to the position and power.
Integrity is not abundant in the US government. That much has become entirely clear to anyone who has been paying attention.
See post 25. They see them as allies, more like. They keep up the pretense of a two-party system in order to hold on to power and push a Fabian means of forcing socialism on the country.