Clive Lewis has called for a referendum on the royal family while pitching to be leader of the Labour party.
The Norwich South MP and shadow Treasury minister called for the poll as part of a package of sweeping constitutional reforms, including the introduction of proportional representation.
Speaking at a campaign event in south London, Mr Lewis said: "Why not have a referendum in this country on the future of the royal family?
"We are a democracy. I'd rather see us as citizens rather than subjects in the 21st century."
He added: "Let's look at what a modern state looks like and what the role of the royal family would be."
Both the shadow Treasury spokesman and shadow foreign secretary Emily Thornberry face a race against time to remain in the battle for the top job, with the deadline for nominations from MPs and MEPs looming on Monday.
The latest figures from the Labour Party showed that Mr Lewis and Ms Thornberry are the only two in the six-candidate field who have failed to reach the threshold of 22 nominations.
Mr Lewis has four signatories - 18 shy of the target figure. Ms Thornberry has nine.
Mr Lewis says if he does win the nomination he'll campaign for a fairer electoral system.
"We must come out in favour of proportional representation, not only because it is the fairest way to elect a parliament, but also because it would put into practice our fundamental belief in the value of collaboration and cooperation," Mr Lewis said.
Shadow business secretary Rebecca Long-Bailey, Lisa Nandy and Jess Phillips all secured the 22 backers required to continue in the competition on Thursday.
As of Friday, Ms Long-Bailey currently has 26 supporters, Wigan MP Ms Nandy has 24 and Ms Phillips has 22.
Early front-runner Sir Keir Starmer, the shadow Brexit secretary, was the first to cross the line earlier this week.
He stretched his lead over the rest of the pack, with the party confirming that, as of Friday, he has 63 backers in the Parliamentary Labour Party (PLP).
Struggling Mr Lewis, who is on the left of the party, will use a speech in south London to lay out his plans for the party if he does win the contest to replace Jeremy Corbyn as leader.