George W. Bush smiles as his brother Jeb recalls a story about their childhood Monday, when the former president Bush hit the campaign trail for his brother in North Charleston, S.C. (Alex Holt/For The Washington Post)

More than a decade later, the war in Iraq is still haunting those who supported it — especially those running for president.

In the Democratic contest, Sen. Bernie Sanders of Vermont has pried open the door on Hillary Clinton’s 2002 vote in support of the war. Despite her public apology for a vote she now says was a mistake, Sanders has said repeatedly that he got that vote right and she did not.

On the Republican side, businessman Donald Trump is going after several of his rivals for their support of former president George W. Bush’s decision to lead the nation to war. Trump’s top target? Bush’s brother, former Florida governor Jeb Bush.

Years after President Obama closed down the war and brought American ground forces home, Iraq remains unfinished business, both as an ongoing national security concern and an open political question that the two parties answer differently.

The Islamic State terrorist syndicate was born out of Iraq’s political and security vacuum. The country is still unstable despite billions in U.S. support. Much of the current debate among Democrats and Republicans looks ahead to the question of who has the better plan to confront that threat. But a surprising amount of the discussion in recent days has also looked back to whether the war was justified and whether U.S. power was misused.

Donald Trump just went after President George W. Bush – and Jeb Bush wasn't having it. (CBS)

Clinton tried to expunge the record that helped sink her first White House campaign nearly a year before she launched her second. She apologized for voting to approve the war in Iraq, called it a mistake and sought to close the door on what seemed a long-ago controversy.

Sanders now touts his 2002 floor speech against the war as a prescient moment, when he spoke about how an invasion of Iraq could lead to a destabilized Middle East for years to come. San­ders was then a member of the House; Clinton was a senator representing New York.

During a rally Monday in Ypsilanti, Mich., Sanders noted that his critics often knock him for the cost of the initiatives he is proposing, including free college tuition at public colleges and universities.

“When we went to war in Iraq, the trillions we spent there, not a problem,” Sanders said, suggesting his agenda shouldn’t be either.

For Clinton, Iraq shadows her central argument that she has the experience and acumen to make the right call on risky and consequential questions. She frequently tells audiences that among all the candidates running this year, she alone has sat in the White House Situation Room deciding matters of war and peace.

“I do not believe a vote in 2002 is a plan to defeat ISIS in 2016,” Clinton said Thursday during the most recent Democratic debate, referring to the Islamic State.

Her campaign and the Correct the Record super PAC supporting her have also asserted that Sanders has little or nothing else to talk about, given his lack of foreign policy experience.

Sen. Bernie Sanders (D-Vt.) emphasized differences between himself and opponent Hillary Clinton and underscored his opposition to the U.S.-led war in Iraq at a campaign event in New Hampshire. (Reuters)

Clinton’s vote on Iraq does not appear to be at the top of most voters’ minds. But given her poor showing in New Hampshire and how close the race is in the upcoming Nevada caucuses, neither does her argument that she is better prepared to be commander in chief.

Less clear is the impact of Trump’s words.

[A tightening race in Nevada threatens Clinton’s post-N.H. ‘firewall.’]

Trump has said that he first opposed the Iraq invasion in 2003, but no evidence has surfaced to support his claim. Nonetheless, he has kept up a drumbeat against George W. Bush’s foreign policy that appeared timed to coincide with the former president’s appearance Monday on the campaign trail on behalf of his brother. The attacks began at last Saturday’s GOP debate in Greenville, S.C., and continued Monday, as if Trump were trying to guarantee that Jeb Bush does not reap the rewards of his brother’s support without getting mired in a public discussion of Iraq.

Trump held a news conference hours ahead of the Bush event and told reporters that George W. Bush was partly to blame for the terrorist attacks on Sept. 11, 2001.

“Excuse me,” Trump said. “The World Trade Center came down during the reign of George Bush, right? It came down. . . . We weren’t safe.”

Trump said the war in Iraq led to instability in the Middle East that gave rise to the Islamic State. “Saddam Hussein was a bad guy,” he said. “One thing about him: He killed terrorists. Now Iraq is [a] harbor for terrorism.”

At the event for his brother, George W. Bush spoke movingly about becoming “something no president should want to be — a wartime president,” but he steered clear of the larger subject of the Iraq war and its legacy.

Jeb Bush came to his brother’s defense. Of Trump’s comments about the Sept. 11 attacks, Bush said, “If I closed my eyes, I thought it was Michael Moore on the stage,” referring to the liberal documentary filmmaker.

[George W. Bush returns to the campaign trail in South Carolina]

Trump’s criticism is out of line with attitudes most Republicans expressed in polls during Bush’s presidency — and since. In South Carolina, which holds its GOP primary Saturday, polling shows that George W. Bush is viewed favorably by 84 percent of Republicans.

Frederick R. Welch, 69, a retired Air Force officer from Myrtle Beach, came to hear both Bushes speak Monday. Trump’s broadside against the Iraq war rankled him.

“It’s very easy once the war has begun to sit back and Monday-morning quarterback,” he said. “We had bad intelligence, but we didn’t know we had bad intelligence. In retrospect we can say it was a bad decision, but that’s not fair.”

A Washington Post-ABC News poll in May found that more than three-quarters of Democrats said the Iraq War was not worth fighting. Among Republicans, 54 percent said the war was worth fighting.

A January 2007 Washington Post-ABC News poll found 83 percent of Republicans thought Bush had made the country “safer and more secure,” while 17 percent said he had not. Later that year, a Post-ABC poll found that 85 percent of Republicans approved of the way Bush handled the U.S. campaign against terrorism, while 12 percent disapproved.

During the debate, Sen. Marco Rubio of Florida skipped past the details of Iraq, and Jeb Bush’s complex family loyalty, to express the gratitude many Republicans feel for George W. Bush.

Many Republicans say, against evidence, that Iraq held weapons of mass destruction when America invaded in 2003. In 2012, a poll conducted by a Dartmouth political scientist found that 63 percent of Republicans still thought this. Last year, a poll from Fairleigh Dickinson University’s Public Mind found a majority of Republicans — 51 percent — believed that weapons of mass destruction had been found in Iraq.

As the 2016 contest began, Iraq was a bigger stumbling block on the Republican side of the race.

Jeb Bush tripped most famously last May, when over the course of four days he struggled to articulate what he would have done about the war given what was now known about Iraq’s weapons capabilities.

“I don’t go out of my way to disagree with my brother,” he said after conceding: “Knowing what we now know . . . I would not have gone into Iraq.”

O’Keefe reported from North Charleston, S.C. Scott Clement in Washington and John Wagner in Ypsilanti, Mich., contributed to this report.