Yet Tuesday is less about the number of delegates to be won and more about the likely effect on the decisions of 'super delegates.'  The proportional allocation of delegates is based on an odd calculus, which mixes district preferences with the statewide popular vote, meaning that it will be nearly impossible for either candidate to win significantly more delegates than the other. Congressional Quarterly reports that a 6% victory in Indiana could actually translate to a single delegate victory.

There are three possible outcomes on Tuesday:

• The first is a Clinton victory in both states, allowing her to reclaim front-runner status. But Obama's earlier victory in Virginia and the state's demographics suggest he will prevail there. Polls put his lead around 5% to 15%; a high single digit margin of victory appears most likely.
• The second is an Obama sweep, which would ensure a wave of support from the remaining super delegates and deliver a knockout blow to the Clinton campaign. Yet Clinton holds a lead in many of the most recent Indiana polls.
• The third -- and most likely -- scenario is that the states are split resulting in Obama remaining the likely nominee and Clinton hanging in the race hoping for an opening.

The pressure is on Clinton. She needs to take around two-thirds of the remaining popular vote in the final six caucuses and primaries to pull even in delegates with Obama, which is highly unlikely. She would also need a perfect storm of Obama faux pas and high profile endorsements to swing enough super-delegates to her camp.

Obama's inability to deliver the coup de grace will worry some Democrats and reinforce the fear that he is too inexperienced a campaigner. He will continue to be attacked by Clinton and expend more money, time and energy on the primary races, when he would prefer to commit resources to securing the centre ground in the general election.

In the meantime, John McCain will organise, tin-rattle, hone his message and take advantage of a splintered opposition. This round of Democratic primaries may see a Republican winner.