They are allies – united in their resolve to oust the CPI(M)-led Left Front from Writers' Building next year.
But they're also engaged in an intense game of hardball over turf, vote bank and, ultimately, political influence. And neither side is willing to blink.
Congress General Secretary Rahul Gandhi's revive Congress project, which paid rich dividends in Uttar Pradesh in the 2009 Lok Sabha polls, and which is making headlines in neighbouring Bihar, has clearly rattled alliance partner Trinamool Congress in West Bengal.
Having replaced the Congress as the principal opposition party across south Bengal, the TMC is now focusing on spreading its wings in north Bengal, where the Congress is still relatively strong.
"I am not a bird of passage, who will just come and go," TMC chief Mamata Banerjee said New Jalpaiguri on Monday in a function to inaugurate and lay foundation stones of 21 railway projects.
Off the record, TMC leaders admitted that she was referring to Gandhi, who had addressed a tremendously successful rally in Kolkata, at which, significantly, he did not once refer to his party's senior partner in the state.
And 48 hours after Banerjee's comment, Gandhi, on his second tour of West Bengal in as many weeks, said: "The prestige of the Congress is a pre-requisite for any alliance."
During last year's Lok Sabha polls, Banerjee had not given the Congress a single seat in south Bengal or anywhere near Kolkata. The closest Congress constituencies were in Purulia and Bankura, both overnight train journeys away from the state capital.
This had caused much heartburn in the mother party (Banerjee had left the Congress in 1996 to set up TMC and a majority of its leaders are former Congressmen).
In the Kolkata Municipal Corporation and other local body elections that followed, the two parties fought separately, but the results clearly proved that public support was with the TMC.
The two parties subsequently came together in several hung municipalities to form local-level coalitions.
Congress leaders rule out any possibility of Gandhi advocating a go-it-alone strategy in West Bengal, as he had done in Uttar Pradesh and Bihar.
"The immediate priority is to defeat the CPI(M) and drive the Left Front out of power in West Bengal," said a senior state leader on condition of anonymity.
But sources said Gandhi has identified West Bengal a focal point of Congress rejuvenation, and was personally overseeing the Youth Congress drive to regain its mass base in the state.
A day before his Kolkata rally on September 6, West Bengal Pradesh Congress President Manas Bhuinya held a press conference calling for all leaders and workers who had defected to Trinamool to return.
"There is tremendous enthusiasm in Congress workers and people of Bengal. I appeal to everyone to join the Congress and those who have left us to return. They will be given due respect," Bhuinya had said.
TMC responded by holding the first conference of the Trinamool Youth Congress in Bhuinya's own constituency Sabang in West Midnapore. TMC leaders like Somen Mitra and Subrata Mukherjee, both of whom had defected from the Congress, took on Bhuinya in their speech, calling him a useless politician.
Even before these recent tensions, TMC leaders had begun criss-crossing north Bengal to try and expand their party's base at the expense of the Congress.
"The Trinamool Congress under Mamata Banerjee is the only party, which can single-handedly bring and end to CPI(M) rule. No one else can do this. Our supporters and workers are with us and will remain with us," said Suvendu Adhikary, TMC MP.
Banerjee's party admits that Gandhi's repeated forays into its new strongholds in south Bengal can eat into its support base.
"The Congress is bringing in Rahul to strengthen itself in south Bengal, where it is marginalised. We are not openly fighting against them, but we have launched our own drive to protect our base," said a senior TMC leader.
In this era of coalitions, managing friends is becoming as demanding as fighting political rivals.