With Nitish Kumar projecting a secular and progressive image, the Congress wants to support at the head of a "secular alliance" to defeat the Bharatiya Janata Party (BJP).
The Nationalist Congress Party (NCP) is all for Nitish Kumar.
This has put pressure on Lalu Prasad to take a call - either to accept Nitish Kumar as the leader of a common alliance or go it alone.
After the Congress publicly decided to ally with the JD-U, NCP leader Tariq Anwar supported the idea.
The Congress and NCP support has given an advantage to Nitish Kumar vis-a-vis Lalu Prasad.
"Nitish Kumar's image as one who has tried to develop Bihar and provided good governance has forced the Congress to side with him," said Congress leader Ashok Choudhary.
"The Congress has to project someone who can aggressively counter (Prime Minister) Narendra Modi's development plank in the polls," he added.
On his part, Nitish Kumar made it clear on Monday that the JD-U would go for a tie-up with the Congress.
"We enjoy very good relations with the Congress. It has been supporting our government," he said. "We want it to contest the election in alliance with us."
Nitish Kumar's comments come at a time when RJD leader Raghuvansh Prasad Singh has come out against projecting Nitish Kumar as the chief ministerial candidate of a possible RJD-JD-U alliance.
The Congress threw its weight behind Nitish Kumar after its vice president Rahul Gandhi preferred the JD-U leader over Lalu Prasad.
"Even ahead of the Lok Sabha polls (last year), Rahul was inclined to go with Nitish Kumar. But (Congress president) Sonia Gandhi opted for Lalu," said another senior Congress leader, Ashok Yadav.
Sonia Gandhi also wants an alliance with the JD-U but she is equally keen to partnering with Lalu Prasad so as to consolidate all anti-BJP votes.
The BJP is divided over who its chief ministerial candidate should be.
JD-U leaders feel that the Congress support to Nitish Kumar will provide him bargaining power when Nitish Kumar talks to the RJD.
"With the Congress and the NCP with Nitish Kumar, the JD-U will bargain hard with the RJD and even look for alternative political alignments minus the RJD for the election," JD-U leader Sanjay Singh said.
According to some JD-U leaders, the Congress still retains strong support among minorities, upper castes and Mahadalits.
"In the last Lok Sabha election, the JD-U contested alone and secured only 15.5 percent of votes. But the RJD contested along with the Congress and performed much better than us," said a JD-U leader.
Congress sources said the JD-U and the RJD were expected to contest at least 100 seats each and leave 43 seats for the Congress, the Left parties and the NCP.
The JD-U won 115 seats in the last assembly polls when it was allied with the BJP. The BJP won 91 seats. The RJD got 22 seats and the Congress only four.
However, differences over Modi's elevation in the BJP led to the JD-U walking out of the alliance in 2013.
The Bihar election will be the first popularity test after the Modi government completed its first year in office in May.
The results are expected to have a wider political resonance amid efforts by Rahul Gandhi to brand the Modi government as "anti-farmer" and "anti-poor".