In separate sectors of the English-speaking democratic world, the capacity of the two-party system to cope with the pressures of an economic crisis is being tested this week, with important implications for both Britain and the United States.
Thursday's voting in England, Scotland and Wales will determine whether it is possible to assemble a majority in Parliament for either the Labour government that has ruled for 13 years or the Conservatives, who have furnished the strongest consistent opposition. The rise of the third-force Liberal Democrats creates the possibility of a hung Parliament and a protracted period of inter-party bargaining.
Meanwhile, four years after Sen. Joe Lieberman showed in Connecticut that a prominent national Democrat could prevail at the polls as an independent, Charlie Crist, the incumbent Republican governor of Florida, has decided to attempt to duplicate that feat in the U.S. Senate race after losing the support of his party.
If either of the third-party or independent challenges succeeds on either side of the Atlantic, it would clearly signal to other ambitious politicians that old loyalties of the two-party era have been so weakened by the combination of modern media politics and tough economic times that they cannot prevail.
For Crist to win in Florida and Liberal Democrat Nick Clegg to gain enough seats in Parliament to be able to bargain his way into some form of coalition government would accelerate what is already a looming crisis for the two-party system.
The symptoms of this weakening, at least in the United States, are the rising intolerance for ideological differences within each of the old parties and the growing distance between their activist wings.
In a brilliant essay updating the work of David Brady and Hahrie Han, the Brookings Institution's William Galston has shown that in the current Congress, the polarization of the parties has become more complete than at any time in the modern era. In both the House and the Senate, the most conservative Democrat is more liberal than every Republican and the most liberal Republican is more conservative than every Democrat.
The gap between the parties, Galston writes in the April issue of Brookings' governance studies, has grown so great that "if one defines the congressional ‘center' as the overlap between the two parties, the center has disappeared."
The polarization has been fostered by the potency of the elected leadership within each party caucus and by the increasingly centralized role of fundraising by each party machine.
But at bottom, it has been fed by the growing partisanship of the core constituencies of the old parties. Their media habits and even their dwelling places also show less of the overlap in tastes.
But as the activists and the officeholders display increasing partisanship, the large mass of relatively indifferent voters who do not follow politics intensely but judge the parties on the basis of their sense of overall satisfaction have become more uncomfortable in what they regard as the two-party straitjacket.
The candidate who seeks to exploit that hunger for something different can be a relative newcomer like young Clegg or someone as familiar as Lieberman or Crist.
In normal times, as voters come closer to the moment of casting their ballots, they become more conventional in their choices. Independent presidential candidates such as John Anderson and Ross Perot struggle to hold the same percentage of support they enjoy in early polls — and almost invariably fail.
But these are not "normal" times. The ranks of the disaffected have exploded over proposals on health care, immigration and other issues, targeting Republicans, Democrats and politicians in general as the mood strikes them. The possibility of catching that electoral lightning in a bottle is what gives those running as independents or third-party leaders their hopes.
Britain offers the first test of that rebellion, but it will not be the last in this volatile year.
E-mail nationally syndicated columnist David Broder at firstname.lastname@example.org.