It fractures the ability of republican leaders to keep their caucus together on issues and votes.
Its a party plank, the goal of this is not to get revenue, this proposal will net around 80billion/year. The odds that this will be mitigated by tax avoidance remain high. Even the President admitted this is not about revenue its about "fairness". If its about fairness, eliminating tax avoidance is a much more surefire way of accomplishing it.
There are a ton of ways to raise revenue that republicans are not just willing to go to but WANT to go to. By insisting on this one they are showing its about politics and not financial responsibility.