Al-Jaafari told the British newspaper the Guardian that he was rejecting calls to give up the nomination of his Shia bloc "to protect democracy in Iraq."
"There is a decision that was reached by a democratic mechanism and I stand with it," he told the newspaper. "We have to respect our Iraqi people."
Nevertheless, pressure is mounting on al-Jaafari to step aside, even from within his own Shia political alliance.
The Iraqi vice president has called for the prime minister to step aside so a new government can be formed, becoming the most senior Shia official publicly to endorse demands for a leadership change to halt the slide toward civil war.
Adil Abdul-Mahdi told the BBC on Tuesday that he had met Ibrahim al-Jaafari and urged him to give up the nomination for a second term because he had lost the confidence of the Sunnis and Kurds.
But Abdul-Mahdi said al-Jaafari refused, insisting he wanted to take his case to parliament, which must approve the new prime minister and his cabinet by a majority vote.
Asked whether al-Jaafari should withdraw his nomination, Abdul-Mahdi said: "Yes, after such a time of naming him, not getting approval from others now in UIA (the dominant Shia political bloc), there is some rejection so I think he should step aside."
Abdul-Mahdi lost the prime minister nomination to al-Jaafari in February by a single vote at a caucus of the Shia bloc, which won the most seats in parliamentary elections last December.
Al-Jaafari squeaked through largely because he had the support of the powerful anti-American Shia leader Muqtada al-Sadr.
On Tuesday, about 2,500 people marched in support of al-Jaafari in Baghdad's Sadr City district, carrying banners saying "Down with the Conspiracy" against their candidate.
Shia officials have been meeting for several days with Sunni and Kurdish leaders urging them to soften their opposition to al-Jaafari.
"I think tomorrow, or the day after things will ease up," said Shia politician Khudayer al-Khuzai, who backs the prime minister.
"They were understanding the grave repercussions of changing. If al-Jaafari is changed, then the (Shia) alliance will split. We cannot abandon him because he was democratically elected."
A showdown is looming between
al-Jaafari loyalists and critics
In a related development, Al Jazeera reported quoting Iraqi security sources that US soldiers had withdrawn heavy weapons from all Iraqi police centres in the cities of Najaf, Karbala, Diwaniya and Amarah, in southern Iraq.
The sources said the US move was prompted by concerns that the weapons might be used in anticipated clashes between Shia militias supporting al-Jaafari and those opposing him.
Also on Wednesday, Al Jazeera aired an interview of Harith al-Dhari, secretary-general of the mainly Sunni Association of Muslim Scholars, in which he blamed al-Jaafari's government for what he called "the killings of over 40,000 Iraqi Sunnis".
Appearing on the Midday Guest programme, al-Dhari also held US soldiers responsible for the deteriorating security situation in Iraq.