Sundering the party from within, proving to the rest of America they can't govern.
In the wake of the 2012 elections, establishment Republicans did the math and realized that they were through as a national party unless they could begin garnering support among growth demographics. It was a simple, objective truth: without Latino, Asians, young voters and women, Republicans were headed to oblivion.
We laugh at GOP efforts at "rebranding", and it was certainly funny how teabaggers put the kibosh on them. Yet they've been unable to effectively kill immigration reform, so it festers within the GOP caucus: the realist reformers on one side, and the ideologue reactionaries on the other. And neither side has the heft or political juice to impose its will on the other. They are effectively paralyzed into inaction.
It's a curious state of affairs. Republicans have two options: 1) kill the bill, and quickly, or 2) pass the bill, and quickly. Either approach will deliver political pain, so like a bandaid, its best to rip that thing off as quickly as possible, then hope that people's attention spans get pulled to the next shiny object (and odds are, they will). Instead, we have this limbo situation delivering the worst of all worlds. Every day this battle drags on, the Republican coalition frays a little more from within, Spanish-language media bashes the crap out of Republicans for their hostility and inaction, and Republicans look incompetent and unable to govern to the broader public.
For a party that set out to reform immigration in a bid to defuse Latino and Asian hostility toward them, their current trajectory is only exacerbating their problem. For a party that needs to come together for the 2014 elections, we're seeing the widest gulf yet, and for a party that needs to prove its governing bona fides for the coming elections, well, they're merely reinforcing every negative stereotype about them.
This isn't even about "Democrats v Republicans" gridlock. If Republicans can't even manage their own house, how are they supposed to manage the U.S. House, much less the Senate?
Senate Minority Leader Mitch McConnell will lose his election—whether in the primary or general is irrelevant—based on voter frustration with Washington gridlock. For a party that has lost America on virtually every single issue of note, losing the public's faith in its governing abilities is the final straw. Without even a veneer of administrative competence, what's left? A bunch of crazed, rigid, out-of-the-mainstream psychopaths hell-bent on making people's lives more difficult.
The last eight months have shown that Republicans have actually done a great job rebranding. It's just not the brand they were hoping to have.