Long-serving SNP MP Angus MacNeil said if "all legal referendum avenues are blocked" then the party "must be able to use the ballot box" to win a mandate.
The Western Isles MP, together with SNP councillor Chris McEleny, are pushing for their alternative path to independence to be discussed at the party's next conference - a one-day event in Aviemore in June.
Both Mr MacNeil and Mr McEleny agree that by the time the conference takes place there will be "no realistic prospect of a referendum happening in 2020" - the timescale First Minister Nicola Sturgeon has set out.
On that basis they argue the 2021 Holyrood election "cannot be fought just to gain the same mandate that's been ignored repeatedly without the SNP having any plan B to ensure the Prime Minister has a political price to pay".
Both the current PM Boris Johnson and his predecessor Theresa May refused requests from the First Minister for powers to be transferred to Holyrood to allow a fresh independence referendum to take place.
Mr MacNeil and Mr McEleny believe if the UK continues to block a second referendum, and if it is not possible for the Scottish Government to hold a consultative ballot, then a pro-independence majority in the Scottish Parliament in 2021 should become a mandate for Scotland leaving the UK.
Pamela Nash, chief executive of the pro-UK campaign group Scotland in Union, branded the plan "yet another direct challenge to Nicola Sturgeon".
But Mr MacNeil said: "There has to be mechanism for Scots to have a view on independence.
"A Section 30, or Plan A, is everybody's favourite but like it or not that has a Boris-veto which he is currently exercising.
"A consultative referendum must be explored before the Holyrood elections to see if that strategy is valid and possible, needing to certainly go through Parliament and possibly the courts."
He added: "If all legal referendum avenues are blocked then we must be able to use the ballot box and an election for a mandate.
"I suppose there is a change of emphasis from last year but what it does ensure is the ultimate safety net that blocking referendums on Scottish independence are not consequence-free for the Tory London Government, which is supported by only a quarter in Scotland."
His comments come after some activists within the SNP have grown impatient with the party's leadership on the issue, amid repeated rejections from Downing Street to grant the powers needed to hold another referendum.
Senior figures in the party, including the First Minister and Pete Wishart MP, have urged patience from their grass roots members.
Ms Sturgeon said it is necessary to ensure the legality of any future referendum so the result cannot be questioned.
But the First Minister has not ruled out the possibility of testing in court the powers of the Scottish Parliament to legislate for a referendum, although she has indicated this would be a last resort.
Mr McEleny, the leader of the SNP group on Inverclyde Council, said: "It is our preferred choice to achieve our independence through a referendum but if we are denied the right to hold a referendum on our future then we must take our destiny into our own hands.
"The consented referendum policy of the SNP is not integral to our national cause.
"In 2021, a parliamentary vote for the SNP should be a vote for Scottish independence."