FYI Communists have already started campaigning for this.
First buddha said that Manmohan is weak and congress is getting weak because of him. Why is he so concern about congress? Then he said today again that Manmohan is a waste.
Their idea is if there is a chance of third front after election then make Communist guy as a prime minister. God bless India!
If there is no chance for third front and it is almost certain that congress wont get more seats than it currently has (in next general election) then congress need support from communists. So Sonia can say communists wont support Manmohan so we will make Rahul as prime minister.
After all what communists gain by opposing Manmohan? Could be that Manmohan is more towards friendship with USA. Or Sonia must have fed them well to say that.