One of the most senior leaders of Saddam Hussein’s government and the highest-ranking figure to have eluded capture after the American invasion in 2003 has died, according to Iraqi officials and the deceased dictator’s Baath Party.

The party said on Sunday that Izzat Ibrahim al-Douri, a close aide to Mr. Hussein, had died in Iraq a day earlier. Muqtada al-Sadr, a populist Shiite cleric and a leading political figure in the country now, tweeted about Mr. Douri’s death, wishing on him “hell and a bad destiny.”

The Baath Party did not give a cause of death. But Mr. Douri suffered from leukemia and received treatment for it for years, according to Essam Alrawi, a commander and a militant in the Baath Party, who spoke in an interview from the Jordanian capital, Amman.

After the United States invasion of Iraq in 2003, Mr. Douri was dubbed the “king of clubs” in the deck of cards that sought out the most-wanted members of Mr. Hussein’s government. American officials, believing he was behind a spate of attacks, offered a $10 million reward for information on Mr. Douri’s whereabouts.

Mr. Hussein’s daughter, Raghad, said in a post on Twitter announcing the death that Mr. Douri was “the brother and real supporter for my father during his rule and after.”

Mr. Douri, 78, was Mr. Hussein’s right-hand man and part of his inner circle in a government that dealt brutally with Iraqi civilians and unleashed catastrophic regional wars.

He was the secretary-general of Mr. Hussein’s once-powerful but now banned Baath Party, which called him a passionate and faithful leader who believed in the right of his people to a dignified life. Even though it is banned, the party has continued to send emails and messages on social media occasionally.

“Today, the knight of the Baath and the Iraqi national resistance has dismounted from his horse,” the statement read, adding that Mr. Douri was a “symbol of courage, heroism and sacrifice.”

The Iraqi Parliament also noted Mr. Douri’s death in a statement issued Thursday in which it condemned a Tunisian politician for demanding that a chapter from the Quran be read in Mr. Douri’s honor. The Iraqi Parliament called Mr. Douri “a fugitive from justice” and said he was involved in the killing of thousands of Iraqis.

Mr. Douri was one of the most trusted allies of Mr. Hussein, who ruled Iraq with an iron grip for almost a quarter-century. Under his rule, Mr. Hussein crushed dissent, fostered a personality cult among Iraqis and cracked down on uprisings by the country’s Shiite majority and Kurdish minority.

He invaded neighboring Iran and Kuwait — two wars that proved catastrophic for Iraq’s economy and led to the death of hundreds of thousands of people. Mr. Hussein presided over a government that was dominated by the Sunni minority and the Baath Party.

Following the Sept. 11, 2001, attacks, the United States invaded Iraq and Mr. Hussein was eventually captured and sentenced to death. Baath Party members and many other Sunnis were kicked out of government jobs — deepening tensions between the country’s Shiite majority and Sunni minority.

For nearly 25 years, Mr. Douri served in several high-ranking government and military posts, including agriculture and interior minister. Rights groups accused him of using excessive military force numerous times, including carrying out a “policy of mass murder” against Iraq’s Kurdish minority in 1988.

Mr. Douri had been reported dead before, once in 2005 and again in 2015. But he continued to appear in audio and video recordings after that. While his wife and daughter had been arrested, Mr. Douri had evaded capture and assumed the role of Baath Party leader after Mr. Hussein was hanged in 2006.

In recent years, he also rose as a leader of the Naqshbandi Order, an insurgent group that included loyalists of Mr. Hussein and joined forces with the Islamic State as it conquered vast territory across Iraq.

While in hiding, Mr. Douri urged Sunni Muslims to rise up against the Shiite-dominated government of the previous prime minister, Nuri Kamal al-Maliki.

Mr. Douri was back in the news recently after a former interior minister under ex-premier Ibrahim al-Jaafari warned that Mr. Douri had been helping to plot a potential military coup.

Mr. Douri was born on July 1, 1942, in the town of al-Dour in Salahuddin Province near Mr. Hussein’s hometown of Tikrit. He grew up poor and worked as an ice seller.

He married five times and had 24 children: 13 daughters and 11 sons.

Mr. Douri’s death reverberated among Baath Party supporters across Iraq and the Middle East. Hassan al-Alawi, a former lawmaker, said his death was “a big loss” for the party and its leadership.

Abdi Latif Dahir reported from Nairobi, Kenya, and Falih Hassan from Baghdad.