Lawmakers will convene at noon (1000 GMT) for their 46th attempt to elect a president but the first expected to actually produce a result.
A majority of parliamentarians from across Lebanon’s deeply divided political scene are set to vote for former army chief Aoun.
The 81-year-old Christian leader has won the support of two of his greatest rivals: Samir Geagea, leader of the Christian Lebanese Forces, and Sunni former premier Saad Hariri.
Hariri described his surprise endorsement of Aoun as necessary to “protect Lebanon, protect the (political) system, protect the state and protect the Lebanese people.”
Aoun already has the endorsement of the powerful Shiite movement Hezbollah, his ally since 2006.
Under a power-sharing agreement, the presidency is reserved for a Maronite Christian while the premier is a Sunni Muslim and the speaker of parliament a Shiite Muslim.
Lebanon’s presidency has been vacant since May 2014, when Michel Sleiman’s six-year term expired.
Since then, parliament has held 45 failed sessions to elect a successor, each time failing to make a quorum because of boycotts by the 20 members of Aoun’s bloc who insisted that he be elected, with Hezbollah’s 13 members also keeping away.
On Monday, a first round will be held to see if Aoun can win a two-thirds majority of 86 votes.
If not, an additional round requires him to win only a 50-percent-plus-one majority of 65 votes, which now looks assured.
His biggest adversary in parliament will be Speaker Nabih Berri, who has said the 13 votes from his Amal party will not go to Aoun.
In Christian-majority areas around the country, Aoun’s supporters have strung up banners hailing him as “the strong president” who can “work miracles”.
Parliamentarian Alain Aoun — Michel Aoun’s nephew — refers to the presidential candidate as “patient, stubborn, and persevering”.
Referring to him by his nickname of “the General”, the MP said Aoun was perfectly healthy despite his age “and has an elephant’s memory”.
Widespread celebrations are expected in Mount Lebanon later Monday, with a huge gathering planned in central Beirut by Aoun’s Free Patriotic Movement.
Most schools and universities have announced they will be closed.
Aoun’s detractors have ramped up criticism of the candidate on social media, accusing him of being a highly strung egomaniac who would ally himself with ancient rivals to get what he wants.
In footage posted by an opponent, Aoun is heard railing against the same parliament set to elect him on Monday as an “illegitimate” body after it extended its own mandate twice.
Even if the presidential vacuum is resolved, analysts say Lebanon’s key political blocs still disagree on almost everything else.