An estimated 12 million voters will be eligible to take part in the day-long polling across 11,763 centres to pick seven MPs from among 150 candidates, including 57 independents.
"We will sweep seven out of seven," said BJP leader Harsh Vardhan, a loyalist of the party's prime ministerial candidate Narendra Modi and who led the opposition when AAP ruled Delhi.
His colleague Vijay Goel told IANS: "There will be a triangular contest but we will emerge on top."
Congress leader Shakeel Ahmed told IANS: "People are disillusioned with AAP and voters who ditched us in December have returned to our fold.
"Our contest is with the BJP," he said, adding quickly that the Congress was not taking AAP for granted.
AAP leader and former Delhi education minister Manish Sisodia is equally sure his party will win all seven seats.
Most political observers, however, feel that the BJP has the clear upper hand in Delhi, and AAP could at best win two of the seven seats.
If campaigning is anything to go by, the BJP is the clear winner.
Delhi is awash with colourful hoardings of BJP's prime ministerial candidate Narendra Modi. The party has also distributed tens of thousands of publicity material.
The Congress has also gone in for heavy advertisement. In contrast, the financially-crunched AAP is virtually absent on the streets and is relying on house-to-house campaigning to reach out to voters.
The Congress won all seven Lok Sabha seats in 2009. But in the December 2013 assembly polls, it got only eight of the 70 seats, a dramatic slide in popular support.
The BJP, out of power in Delhi for 15 years, won 31 seats. AAP finished with a surprising 28 seats, triggering a nationwide interest in the new party.
The Congress candidates include central ministers Kapil Sibal (Chandni Chowk), Ajay Maken (New Delhi) and Krishna Tirath (North West) besides Sandeep Dikshit (East Delhi), son of former chief minister Sheila Dikshit.
The BJP's prominent contestants include Harsh Vardhan (Chandni Chowk), spokesperson and Supreme Court lawyer Meenakshi Lekhi (New Delhi) and Bhojpuri actor Manoj Tiwari (North East).
AAP has fielded Rajmohan Gandhi (East Delhi), grandson of Mahatma Gandhi, and former Delhi social welfare minister Rakhi Birla (North West). Two journalists also figure in the AAP list: Ashish Khetan (New Delhi) and Jarnail Singh (West Delhi).
Polling begins at 7 a.m. and ends at 6 p.m. in a bid to draw more and more voters.
According to officials, the first-time voters number 128,000. A total of 324 voters are over 100-years old.