Many commentators are forming a conclusion that Marian Lupu's defection is a communist project and that the democrats will form a coalition with the communists after the election, the price of which will be handing the Presidency to Lupu.
There is probably some truth to this; whether actively planned or passively allowed, Voronin is probably counting on Lupu's support to shore up his power base after the election. There would need to be a few cosmetic 'democratisation' initiatives (such as we saw with the PPCD in 2005), and Lupu would be the figurehead, but other than that Voronin could conduct business as usual.
But now think from Lupu's perspective. He's a generation younger than Voronin and still has a long political career ahead of him. Why would he want to tie himself to an aging dictator when he could instead rely on the support of the more politically acceptable liberal bloc to achieve the presidency?
More than that, think of the longer term. What Lupu and his backers (both Russian and European) want to achieve is the replacement of the Communist Party by the Democrat Party as the leading party of the Moldovan centre-left.
With the support of the liberal bloc, the PCRM could be eliminated from the Moldovan political scene over the next four years. Its leaders would be prosecuted for their various crimes while the party itself could face severe sanctions for constitutional abuses. Teleradio Moldova would be freed from communist control, allowing eight years of brainwashing to be unwound and grass-roots communist support to fall away.
That would leave the Moldovan political scene from 2013 onwards looking rather normal, with the (mildly pro-Russian) Democrats on the left and the (mildly pro-Romanian) liberal bloc on the right alternating in government. Lupu could look ahead to a long and illustrious political career, relatively untainted by the dictatorship of 2001 - 2009.
[One final comment, so nobody gets me wrong. I am not a supporter of Lupu and do not encourage anyone to vote for him, unless the only other option is the PCRM. Moldovans should vote for the three liberal parties who proved their integrity on June 3rd.]