The Middle Ground
The flaw in the "unity" argument is that it assumes the "middle ground" is between the parties. Thus the need for bipartisanship becomes defacto.
But what if the "middle ground" is not between the parties? What if the "middle ground" is, as Krugman suggests, to the *left* of the Democratic Party? In such a case, a "unity" platform that splits the difference between Republicans and Democrats would, in fact, move the government further away from what the country wants. It is a solution in search of a problem.
This is really not about parties. It is really about moving the government so that it matches the consensus of the American people. The GOP is way out of the mainstream while the Democrats are only mildly off the mark. But both are on the same side of the balance. A "unity" platform that has at its foundational belief the need for bipartisanship between the parties is, in fact, fundamentally flawed. It is guaranteed to move the government even further away from what the people want.
We all want "the middle ground". We just disagree on where that middle ground lies.