Mr Patel, who won the Upper House seat from Gujarat in an election that went down to the wire, said the victory had enthused the Congress and filled the cadre with new energy.
"I am sure we will also win Gujarat. The BJP made it a prestige issue, it is their loss," Mr Patel, who is usually reticent, told reporters at a protest rally at the Jantar Mantar organised by the Youth Congress.
The Congress has been out of power in Gujarat since 1995.
Targeting Prime Minister Narendra Modi and BJP President Amit Shah, Mr Patel accused the two of "misusing" central probe agencies.
"Even the BJP is afraid of these two people. One is a constitutional authority and another is an extra- constitutional authority. You know who these two people are. All agencies are being misused," Mr Patel alleged, without naming the BJP leaders.
In the run-up to the Rajya Sabha election, the Congress accused the BJP governments in Gujarat and at the centre of pressuring its MLAs by using the state machinery. It also herded its MLAs to a resort in Bengaluru.
The income tax department had raided a Karnataka Congress minister, prompting the party to accuse the Centre of "terrorising" its MLAs.
Mr Patel, a Congress heavyweight and political secretary to Congress President Sonia Gandhi, claimed the BJP offered its MLAs Rs 15 crore to change sides, but everyone rejected it.
He said the youth should now give a call against the BJP to "quit the seat of power" (gaddi chodo).
"They promised two crore jobs every year and 50 per cent more profit to farmers than what they spend on their yield.
They promised to bring down inflation and corruption. But they failed on every front," Patel said.
Congress leaders C P Joshi and Raj Babbar also addressed the gathering.
Congress workers led by Youth Congress President Amarinder Singh Raja marched towards Parliament, but were detained by the police.