Many Egyptians now hail him as the nation's saviour after he ousted the Islamists from power and as the only figure strong enough to lead.
Still, if he becomes president, al-Sisi runs enormous risks.
His presidency would enmesh the military even deeper into politics, putting the credibility of the powerful institution on the line if he fails to resolve the country's woes. Turmoil may only increase with a backlash from Islamists, who now despise al-Sisi for his ouster of Islamist president Mohammed Morsi and for the brutal crackdown on their ranks that has arrested thousands and killed hundreds since.
And there is little indication of how he would rule.
Secular critics fear a return of an autocracy similar to that led by Hosni Mubarak for nearly 30 years until his ouster in 2011's popular uprising. Al-Sisi has said it is impossible to now return to Mubarak's style of rule and that the country must move to democracy. But elements of Mubarak's police state — including top security officials and the business elite — are among his fervent supporters, and the crackdown on Islamists has already expanded into a wider suppression of dissent.
Many al-Sisi fans tout him as a new Gamal Abdel-Nasser, who rose to the presidency after the 1952 coup that toppled the monarchy and became a charismatic strongman, inspiring the nation with grand projects like the building of the Aswan High Dam and his vision of Arab nationalism.
A personality cult unseen in the country since Nasser's era has swiftly risen up around al-Sisi. It depicts him as pious and modest, sensitive and emotional yet firm and decisive, a patriot rooted in the common folk of his childhood home el-Gamaliya, a corner of Cairo's Medieval Islamic City seen as embodying the country's best traditions.
Dramatic shows of popularity
As president, that popularity could enable al-Sisi to push through potentially controversial changes. In unpublished footage from an interview with an Egyptian newspaper that was recently leaked, for example, he talks of lifting subsidies on food and fuel "all of a sudden". The subsidies are a gigantic drag on the government budget that economists agree must be reformed — but since Egypt's impoverished population relies on them, attempts at reform has repeatedly fallen apart. (The comments also point to how Nasser comparisons only go so far: Al-Sisi has expressed support for faster privatisation in the economy, reversing Nasser's socialist legacy.)
Conceivably, al-Sisi could even be the only player powerful enough to force reconciliation with Islamists — an idea that has become politically unspeakable amid the fervour to crush the Brotherhood. Some officials have suggested he is a relative dove among more hard-line anti-Brotherhood officers in the military and security forces, though so far he has shown nothing but support for the crackdown on the group, which was launched after giant rallies he called for in July to "mandate" him to fight terrorism.
The dramatic shows of popularity now could mean little later in a divided nation that has turned against two rulers the past three years. The difference this time is the overt commitment that the military, Egypt's most powerful institution, has invested after its top body of generals, the Supreme Council of the Armed Forces, publicly gave its backing to an al-Sisi candidacy on Monday.
"The military's reputation is bound up with an al-Sisi presidency," said Michael Hanna, an Egypt analyst and senior fellow at the Century Foundation in New York.
If al-Sisi cannot stop the mounting insecurity and the further deterioration of the economy, the public backlash could come against the military as an institution, a situation that "if anything, could create divisions within the military," he said.
Al-Sisi knows the dangers. During Morsi's presidency, when Islamists and their opponents were clashing in the streets, he warned in speeches that if the military intervened in the political conflict, it could take decades for it to return to the barracks. In a meeting with army officers late in 2012, he warned that military interference in politics is dangerous for both the state and the army, pointing to Syria, where the military backed President Bashar Assad against an uprising and the nation crumbled into civil war.
"It is not patriotism to take sides ... this is not my business," he said. "The vendetta won't end, the divisions will continue and chaos will persist for two, five, 10 years. The Syrian state is over."
Al-Sisi has not announced that he will run, though Egyptian media trumpet that he will imminently. This week, al-Sisi was promoted from general to field marshal, the military's highest rank, a step seen by many as a final honour before he leaves the military, a step required by law before he could run.
The 59-year-old al-Sisi was the head of military intelligence before he was named by Morsi as army chief and defence minister. His inner circle remains filled with spy masters. Among them is Mahmoud Hegazy, the current military intelligence chief whose daughter is married to one of al-Sisi's sons; Murad Mawafi, the former chief of general intelligence; and Farid el-Tohami, that agency's current head.
The Brotherhood learned the dangers of trying to read al-Sisi.
Morsi elevated him apparently believing he would be dependent on the Brotherhood. A Morsi aide, Wael Haddara, wrote after the coup that al-Sisi was chosen because he was the youngest of the top brass, "so it was hoped he doesn't have cronies and has no real backing" within the military.
Many in the Brotherhood leadership thought he was sympathetic to the Islamist movement, since he was known as a pious Muslim. That belief was so pervasive that one of the most vehement critics of the Islamists in the media, TV personality Tawfik Okasha, at the time warned that al-Sisi was "the Muslim Brotherhood's man in the military." Now Okasha is a fervent al-Sisi cheerleader.
A paper that al-Sisi wrote while studying at the US War College in 2006 shows a political mind well aware of what underpinned Mubarak's police state and of the challenges of creating a democracy.
He says democracy can only come in the Mideast in its own form, meaning anti-US Islamist parties must be allowed to participate and "legitimately elected parties [should] be given the opportunity to govern". He writes that Mideast democracies will have to reflect the region's Islamic culture — "established upon Islamic beliefs," though not a theocracy.
He criticises the region's autocracies for fixing elections, relying on patronage and controlling the media, all features of Mubarak's rule. One obstacle to democracy, he writes, is that police and military in Arab states are loyal to regimes and ruling parties, not the nation.
He concludes that building democracy could take a transition as long as 10 years, and that populations must be prepared by improving education and reducing poverty.
Proponents believe a strongman like al-Sisi is needed to guide that transition.
Al-Sisi would head a broken political system. After Mubarak's fall, Islamists had the strongest political parties, particularly Morsi's Muslim Brotherhood, and dominated elections the past three years. Most of them will likely continue to boycott politics for the foreseeable future. That leaves the field to the fragmented and weak secular-leaning parties, which have little grassroots support.
Little is known about al-Sisi's private life. His wife, Intissar, a cousin he married nearly 30 years ago, has never been seen in the media. The couple has three sons and a daughter. Two of his sons and his daughter graduated from the military college, and the third son works in the Administrative Oversight Agency.
Al-Sisi grew up in Cairo's el-Gamaliya district, a centuries-old neighbourhood of historic mosques immortalized in the novels of Egypt's Nobel literature laureate Naguib Mahfouz. Nearby is the historic bazaar Khan el-Khalili, a main tourist attraction, and al-Sisi's father had a shop selling wooden antiques. The bazaar has been hit hard since tourists largely disappeared in the turmoil since Mubarak's ouster, and its streets are plastered with al-Sisi posters now, with shop owners hoping he can bring stability.
One neighbour, Ali Hossan, who now runs one of the pro-al-Sisi campaign offices, remembered him as "very quiet and not sociable", focused on two things — his studies and helping in his father's shop.
One critic, Ahmed Maher, the leader of the secular youth activist group April 6, which led protests against Mubarak, sarcastically gave his backing to an el-Sissi run in a letter from prison, saying it would fuel a new revolution against the military. Maher was jailed in December to serve a three-year prison sentence for violating a new draconian law virtually banning protests. He blamed al-Sisi for abuses of detained protesters during the anti-Mubarak uprising.
"Let us see the blunt rule of the military instead of the concealed one," he wrote.