DAMASCUS — Syrians are to vote Sunday on a new constitution that could end five decades of single-party rule, although the opposition has called a boycott amid a bloody crackdown on dissent. The newly proposed constitution was drafted as part of reforms promised by President Bashar al-Assad’s government in a bid to calm an 11-month uprising against his regime that began with democracy protests. It is unclear how the ballot can go ahead in parts of the country hit by violence as government forces move in on protest hubs and rebel strongholds such as the besieged central city of Homs. But posters and billboards calling on people to vote are displayed across Damascus, with state television airing non-stop programs from around the country about the new constitution. The opposition has urged voters to stay away and to go on strike, while demanding Assad’s ouster and an end to the crackdown that monitors say has killed more than 7,600 people since March 2011. In Sunday’s referendum, more than 14 million people over the age of 18 are eligible to cast ballots at 13,835 polling stations. Sunni Muslims account for 75 percent of Syria’s population of 22 million, with the rest made up of minorities, including 12 percent for the Alawite community that Assad hails from. They are being asked to vote on the constitution framed by a committee of 29 members appointed by the Syrian president. One of the main changes in the newly proposed charter is to Article 8. It now says Syria’s political system will be based on “pluralism,” dropping its previous declaration that Assad’s Baath Party is “the head of state and society.” This effectively ends the monopoly on power the Baathists have enjoyed since taking over in 1963 following a coup d’etat.