Abdullah, who came second to Karzai in the chaotic and fraud-riddled 2009 election, said late yesterday he hoped the April election would "bring security and prosperity to Afghanistan's people".
The presidential debate, broadcast by the Tolo News channel, yielded few surprises, but Abdullah and four other candidates coalesced on the need to improve the country's dire security situation as US-led NATO troops wind down their presence.
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Gunmen shot dead two of Abdullah's aides in the western city of Herat on Saturday, dealing an early blow to hopes of a peaceful campaign.
Stood on a platform adorned in Tolo's trademark red and blue colours, the quintet responded to two hours of questions from an Afghan journalist, with the format allowing few chances for exchanges between the candidates themselves.
Among the other heavyweights present were former finance minister Ashraf Ghani, Karzai loyalist Zalmai Rassoul and the president's low-profile elder brother Qayum Karzai.
The candidates broadly backed the bilateral security agreement (BSA), which would allow about 10,000 US troops to be deployed in the country after NATO withdraws by December. President Karzai is refusing to sign the pact and wants to leave it to his successor.
Rassoul said the BSA would "bring long-lasting peace to Afghanistan" but Ghani cautioned that "for peace, both sides need to be flexible".
Qayum Karzai said "we need them" in reference to foreign troops, while saying he would sign the agreement, a stance backed by Abdullah.
The debate participants were mostly in step on the need for negotiations with the Taliban while condemning attacks by the fundamentalist militants.
Rassoul said Afghans "who accept the constitution are welcomed" but warned that "those who burn our schools, mosques, behead our soldiers, we have to fight them until their elimination".