Leinen was speaking after MEPs recently rejected a proposal for transnational lists of candidates for seats in the European Parliament.
EU national governments must still vote on the plan ahead of the May 2019 European elections and national leaders will discuss the issue at a summit in Brussels on Friday.
The proposal failed after the EPP group said there is no legal basis for what was labelled “another elite-driven project” by one MEP.
After the UK leaves the EU in March 2019, Parliament will have fewer deputies and the assembly is seeking to reallocate some seats to under-represented countries like Spain, Italy, France and the Netherlands.
French President Emmanuel Macron and European political groups including the Greens and ALDE say transnational lists would help face down eurosceptic parties that have seen strong support in recent years.
After the plenary vote earlier this month, ALDE group leader Guy Verhofstadt said it was “incredible” the EPP had voted against the plan, adding that former Belgian Prime Minister Wilfried Martens, who was a member of the EPP political family, supported it.
Verhofstadt said, “We’ve lost the battle today, but not the war. We’ll keep on fighting for a real European democracy and European citizenship.”
However, several EU leaders have expressed opposition, saying transnational lists would be dominated by France and Germany.
The idea has been championed by Leinen, a German Socialist MEP, who told this website he “convinced” Brexit offers “a unique window of opportunity” for electoral reform.
“We have the unique opportunity to use a part of the British seats for transnational lists so no member state will lose a seat due to their introduction,” he said.
He said the idea was backed “not only by Commission President Jean-Claude Juncker, but also many member states, including France, Spain, Italy, Ireland and Belgium, as well as all southern countries.”
He flatly denied that transnational constituencies would bypass the current link between MEPs and their electorate, saying, “On the contrary, the link would never have been stronger. One person, one vote, no matter where you live. Transnational lists are good for voters and gives them more power at the expense of backroom deals. People would decide, for example, who becomes the next Commission President.”
Critics, such as Manfred Weber, Parliament’s EPP group leader, have said transnational lists could be utilised by populist movements.
Leinen countered by saying, “This is a very defensive argument. So, we cannot win against populist and nationalist movements in a Europe-wide democratic competition? We shouldn’t be afraid of democracy. Transnational lists will be used by parties of all political directions and it is our job to win the hearts and minds of the people by having the better arguments.”
He added, “Populists can only choose the President of the Commission if they win a majority in the European Parliament - which would mean that we did a very bad job. It is the Parliament that elects the Commission President. If this argument is valid, why are all populist and national forces opposing transnational lists?”
He spelled out how such a system would work, saying each list would consist of candidates from at least one third of the member states.
“The share of nationals from one member state must not exceed 25 per cent, the first seven candidates on the list must be nationals from different member states and lists would alternate between candidates of different EU countries.”
He and Verhofstadt have joined forces with other MEPs from across the political spectrum - Pascal Durand, Jerome Lavrilleux, Mercedes Bresso, Philippe Lamberts and Sophi In ‘t Veld - in signing a letter supporting transnational lists.