Will TMC take a hit because Congress, Left on same side?

The second part of the first phase of Assembly elections in Bengal could witness a keen contest between the Trinamool Congress and the Left-Congress combine if the last two polls are a guide to voting patterns.
Of the 31 seats in phase 1B, TMC won 18 in 2011 and its then ally Congress, won three leaving just 10 for the Left. In terms of vote share, the contest was much closer, the Left mopping up 44.5% compared to the TMC-Congress alliance's 48.6%.
The key question now is how the switch of the Congress from the ruling combine to an opposition one changes those equations.Another question on which the outcome could hinge is how the BJP performs. In 2011, the BJP was a really marginal presence in this part of the state, which includes some parts of the Maoist-affected Jangamahal and of Burdwan, a district that's both an agricultural and industrial hub.
By the 2014 LS elections, the BJP had dramatically improved its position, winning almost 19% of the votes and leading in six assembly segments. All but one of these leads were in the Asansol LS constituency, won by Babul Supriyo for the saffron party. Indications are that the BJP's star has faded significantly in Bengal since then, even if its vote share is unlikely to sink to the 2011 level of under 4%.

The imponderable then is who gains from a decline in the BJP's vote? In 2014, the TMC had 43.5% of the votes, the Congress and the Left put together polled 34.3%. That's a gap of just over 10%, which might seem like enough of a cushion. But if the BJP's vote share declines significantly in the absence of a Modi wave and most of that shifts to the opposition combine and if the TMC sees some reduction in its vote share, the cushion may disappear very fast.
Chief Minister Mamata Banerjee would be aware that even in 2011, nine of the 18 seats her party won were with margins of under 10,000 votes.
Those would be vulnerable with a former ally now adding to the rival's tally. On the other hand, if she can hold on to the votes her party polled in the LS elections, she can make such arithmetic largely irrelevant.