In Elections-09, Congress had a vote share of 28.6 % in 2009; up about 2 % over 2004. BJP, on the other hand, ended up with about 18.8 %, down about 3.5 % from 2004.
The vote share difference widened, then, from 4.5 % in 2004 to almost 10 % in 2009.
This statistic, however, is not all that useful, because these two parties contested in alliance with others, so we really ought to look at the performance of these alliances -- UPA and NDA. Also, the number of constituencies in which the two parties (and perhaps the two alliances) contested are likely different.
This statistic also doesn't take into account the diversity of contests in our States. At one extreme, we have straight fights (in Delhi, for example); and at the other, we have multicornered contests (in UP, for example) with multiple players with roughly equal strength.
In Delhi's one-on-one contests, your vote share has to be close to 50 % to win (indeed, Congress did get 57 % against BJP's 35 % or so).
In UP, on the other hand, we had a four-cornered contest. Thus, BSP, SP, and Congress got 27.4, 23.3. and 18.3 %, respectively. They won, respectively, 20, 24 and 21 seats.
In other words, even though BSP had a 4 % and a 9 % lead in votes over SP and Congress, its seat tally was less than theirs! No wonder Mayawati is upset.
In between these two extremes, we have situations where a minor (third place) player whose small vote share is also larger than the difference between the winner and the runner-up. In AP, for example, Chiranjeevi's party played such a role. Or, Raj Thackeray's MNS in Maharashtra. P. Sainath alluded to a similar role by BSP in Vidarbha region of Maharashtra. In many constituencies, rebel candidates also play this role.
I'm looking for some state-wise analysis of vote shares to see how things panned out for the UPA and NDA. Similarly, I'm also looking for some data on how the women, different caste, religious, income and age groups voted in different states. (I'm not yet ready to buy all the glib TV talk about Muslims switching to Congress in UP ...).