‘In politics, again, it is almost a commonplace, that a party of order or stability, and a party of progress or reform, are both necessary elements of a healthy state of political life; until the one or the other shall have so enlarged its mental grasp as to be a party equally of order and of progress, knowing and distinguishing what is fit to be preserved from what ought to be swept away. Each of these modes of thinking derives its utility from the deficiencies of the other; but it is in a great measure the opposition of the other that keeps each within the limits of reason and sanity.’
I think the typical view of politics from inside a partisan mindset is to see politics as a battle of the good guys versus the bad guys. Maybe the good guys are on the left, maybe the good guys are on the right, but it’s this Manichean struggle and the way to get progress is for the good side to win and impose their will. Mill sees through that and sees that, in fact, politics is a dialectical process. At any given time truth is partly on one side and partly on the other. It’s more a battle of half-truths and incomplete truths than of good versus bad. The excesses of each side ultimately create opportunities for the other to come in and correct those excesses.
Partisans might think that everything the other mob do or believe in is irredeemably vile but mercifully most people aren't blinkered partisans and, in fact, are prepared to give a new government a fair crack of the whip. In this, the voters are better than the political parties they're compelled to choose between and this is also why politicians who rise above or move beyond party are the most successful. Blair understood this and so, I think, does Cameron.
Not everything Labour did in office was terrible and, in any case, there needed to be a correction after 18 years of Conservative rule. Similarly this present government was needed to replace an exhausted, fagged-out Labour ministry. None of this is terribly mysterious but it's oft-forgotten amidst the hurly-burly of the Westminster village. It may be that Mill's belief that, eventually, each party improves the other is overly-optimistic but, looking at recent British political history, he'd seem to have been proven correct.
Incidentally, if you haven't read Lindsey's splendid book The Age of Abundance then, well, I recommend that you do so soonest.