The former political prisoner has attended all 20 or so rounds as head of the opposition delegation to the Astana talks, the last of which concluded last month without any progress on ending Syria's 12-year internal conflict.
The talks are a by-product of the Russia military intervention in Syria in 2015, which reversed the balance of power in the war in favour of Assad.
Moscow's move forced the US to acquiesce to a Russian diplomatic approach that focused on ceasefires and undermined previous international agreements on reaching a political solution preceded by a transitional period. Turkey and Iran were brought in as so-called guarantor states.
Mr Tumeh says Russia used the talks to restore Assad's control over Syria by promising reprieves to rebels in besieged areas, then bombing them into submission.
The opposition kept counting on a more assertive Turkish position, which in 2020 saved the last rebel areas in north-west Syria from falling to the régime and its allies, after Turkey and Russia came to the brink of war over Syria.
“Without Astana, the Russians and Iranians would have taken over all of Syria,” says Mr Tumeh, speaking in Istanbul.
A veteran of the mostly underground Syrian dissident movement from the eastern province of Deir Ezzor, he fled to Turkey a year after the peaceful anti-Assad uprising in 2011 escalated into civil war.
He became head of the opposition Astana delegation, composed mostly of rebel commanders, because of his connections with Turkey and his moral sway over forces on the ground.
Mr Tumeh believes the opposition's best chance of extracting any concessions from world powers lies in strengthening relations with Turkey, especially after the US and some Arab countries abandoned support for the rebels in 2015.
Washington maintained support only for Kurdish militias in the north-east, which the US military employed as the main ground component of the war on ISIS in Syria.
The opposition, he says, has not lost any territory since 2020 as Astana became mainly a forum to deal with ceasefire violations and Turkey bolstered rebel military capability in the north-west.
The invasion of Ukraine has also bolstered the position of anti-Assad forces in the area, with Russia reportedly having moved troops and equipment from Syria to Ukraine to prop up their position there.
“The Russians got mired in Ukraine and it became more difficult for them to pressure us to achieve political gains,” says Mr Tumeh, a dentist by training who studied Islamic theology.
In the 1980s, he became a disciple of the late Syrian Islamic thinker Jawdat Saeed, a proponent of non-violence who opposed the fanaticism of some of the Muslim Brotherhood’s ideologues.
Mr Tumeh was sentenced to three years in jail in Damascus in the 2000s for "undermining national morale’" and headed a provisional government, formed in 2013 in Turkey, to fill the vacuum in the areas that fell out of Assad’s control.
The north-west is divided into areas controlled by Tahrir Al Sham, an Al Qaeda offshoot with open channels with Turkey, and the Syrian National Army, formed by Turkey in 2017.
The Syrian National Army is comprised mainly of rebels who were defeated after Russia intervened in Syria in 2015.
Using mainly air power, Russia helped to push back a march by an alliance of rebels and militants from the north-west to coastal strongholds of Assad’s Alawite sect.
Many of those fighters became part of Tahrir Al Sham, the most powerful anti-Assad faction in Syria.
Dealing with Tahrir Al Sham is “complicated”, Mr Tumeh says, especially considering it is widely designated a terrorist group, even by Turkey.
He says “strategic ideological issues” need to change for any chance to regain international support for political change in Syria.
“Without moderating the ideas of all the parties in the conflict, a political solution is not possible,” says Mr Tumeh.
The cafe in the middle-class district of Al Fatih where he spoke is full of young Syrians, with whom Mr Tumeh often debates, as well as on his visits to the north-west and on social media forums.
“Syrians are intertwined with civilisation, progress and love of life. We need to return to our moderate roots,” he says.
Over the past decade, Syria has become divided into zones controlled by ideologically opposed militias, owing allegiance to Turkey, Russia, the US or Iran.
In the past three years, a stalemate between the big players has largely frozen the conflict on the ground as well as rival international diplomatic tracks on Syria – the Astana talks and a UN peace process in Geneva.
In a surprise move, Kazakhstan last month proposed the Astana process should end because of the changing picture of Mr Al Assad's international isolation.
Mr Tumeh said Kazakhstan's move could be related to its ties with Russia, which have been changing amid the Ukraine war. He expected the talks to continue in another location, as Moscow has indicated.
"Kazakhstan's proposal was not accepted," he says.
Russia also is facilitating talks between Damascus and Ankara to re-establish relations and find ways to jointly undermine the Kurdish militias that control large swathes of north-eastern Syria with US support.
The militias are composed of the Turkish Kurdistan Workers Party (PKK) and the People's Protection Units (YPG), a group that traces its origins to the Syrian branch of the PKK, which is designated a terrorist entity by Turkey and a number of other countries.
But Turkey regards the two groups, which allow bases for Syrian intelligence and government operations in their areas, as mortal enemies.
Arab states have also sought accommodation with Assad, culminating in the readmission of Damascus to the Arab League in May.
But Mr Tumeh insists the situation remains in the opposition's favour.
He refers to a more active US posture against Iran in Syria, such as a so-far unrealised American effort to form an alliance of Arab Sunni tribes in the north-east as a buffer against pro-Iranian militias in the area.
The main players in the Arab League, he says, might further improve ties with Mr Assad but "will not support him to recapture Syria, with all of its resources, and resume exporting crisis to the region".
The Arab normalisation is opposed by the US and most western powers. These countries have also resisted Russian efforts to raise international funding for rebuilding in areas controlled by Mr Al Assad and to lift sanctions on his government.
The sanctions were first imposed in response to the state violence used to crush the 2011 demonstrations.
But the US has not sought to make Syria part of the deepening confrontation with Russia over the Ukraine war, Mr Tumeh says.
He pointed out that Washington had made little effort to revive the Geneva track, which is supposed to find a political solution to the war and to produce a transitional governing body in Syria.
The Geneva process is divided into constitutional and political paths, with Badr Jamous, a Syrian businessman with connections in Eastern Europe, leading the opposition delegation on the dormant political track. Hadi Al Bahra, another businessman who worked in Saudi Arabia, heads the opposition group taking part in the sporadic talks on a new constitution, without much progress to date.
Both men, together with Mr Tumeh, are members of the National Coalition of Syrian Revolutionary and Opposition Forces, an organisation based in Istanbul. It was formed with western and Arab support in 2012 to represent the opposition and unite the rebel brigades but has been undermined by divisions from its inception.
Although the Geneva meetings have been symbolic, they remain crucial for the case, Mr Tumeh says.
"We are there to preserve the core of the Syrian revolution," he says. "We cannot allow the Syrian file to turn into a mere humanitarian issue and the political solution forgotten." '
