Iraqi Prime Minister Nouri al-Maliki. AFP
"We categorically reject the blatant American interference in Iraq's internal affairs," Maliki said on X, adding that "we consider it a violation" of Iraq's "democratic system", in place since the 2003 US-led invasion.
Trump on Tuesday said that Iraq would make a "very bad choice" if Maliki were selected for the top post.
He warned that because of Maliki's "insane policies and ideologies, if elected, the United States of America will no longer help Iraq".
Maliki was endorsed as Iraq's next prime minister by the Coordination Framework, an alliance of Shia groups that has been in recent years the main ruling coalition.
In his post, Maliki denounced the US interference saying it "infringes upon the Coordination Framework's decision to nominate" its candidate.
He added that in line with that decision, he "will continue to work until the end, in a way that achieves the higher interests of the Iraqi people".
Maliki, Iraq's only two-term prime minister (2006-2014), fell out with the US during his last stint over deepening ties with Iran and after being accused of pushing a sectarian agenda.
But the 75-year-old shrewd politician remained a central figure in Iraq's politics and a powerbroker whose approval is considered indispensable to any governing coalition.
Short link: