It's notable that Clinton still has a tactical advantage since the national debate has shifted away from economic issues and zeroed in on national security and terrorism -- especially with news Saturday that Iran freed four Americans, including The Washington Post correspondent Jason Rezaian, in a deal that provided clemency for seven Iranians charged or imprisoned in the U.S. for sanctions violations. A December Washington Post-ABC News poll showed more than 3 in 5 voters trust her more than her opponents to handle issues related to terrorism and national security. But her main opponent has found ways to chip away at that, too.
As the Democratic primary gets more muddied than most would have predicted, here are the eight top issues for Sunday's Democratic debate.
1. The U.S.-Iran prisoner deal
World leaders were converging in Vienna on Saturday to prepare to lift sanctions against Iran as part of a historic nuclear deal, when news broke that four Americans imprisoned in Iran would be freed.
The four include a Christian pastor, a former Marine and Rezaian, who had been in captivity in Iran for 18 months and was most recently held in the country's notorious Evin Prison.
In exchange, the U.S. released the seven Iranians imprisoned or charged related to violating sanctions against Iran. A U.S. official in Vienna told The Post that no Iranians charged with crimes related to terrorism were freed.
Upon news of the exchange, most Republican presidential candidates immediately criticized the Obama administration. Some, like Sen. Marco Rubio (R-Fla.), said the deal sets a bad precedent for future relations.
Others, like Ben Carson, former Arkansas governor Mike Huckabee and Sen. Rand Paul (R-Ky.), praised parts or all of the prisoner release while still offering harsh words for the nuclear deal Obama and five other nations signed with Iran in 2015.
Democratic presidential candidates appeared to have a different take on the news. In a statement Saturday, Sanders said the exchange shows "diplomacy can work." Late Saturday, Clinton issued a statement welcoming the release, but adding, “We shouldn’t thank Iran for the prisoners or for following through on its obligations. These prisoners were held unjustly by a regime that continues to threaten the peace and security of the Middle East.”
All three Democratic candidates will have a chance to offer more thoughts on the complicated U.S.-Iran relationship -- and its latest development -- on Sunday night.
2. Sanders's general election viability
Sanders's momentum is raising the possibility among the Democratic elite in Washington that the socialist who wants to give all Americans Medicare, provide tuition-free public college, break up the big banks and tax the heck out of Wall Street actually has a shot at winning the nomination.
The Washington Post's Paul Kane notes that many of Sanders's proposals go beyond the Democratic Party's official agenda and are far to the left of the nation as a whole. But Sanders's success so far could be offered up as proof that the socialist's ideas are being embraced by a growing populist -- and even socialist -- wing of the party.
(A new Seltzer & Co. Iowa poll found that more Iowa Democratic voters identify as "socialist" than "capitalist.")
Clinton and the Democratic establishment are trying to make the case that Sanders is too far left to be the party's standard-bearer in November. But Kane reports they're trying to tread carefully so as not to alienate Sanders's growing voter base.
The question Sanders must answer is whether he's got a broad enough umbrella to be the party's nominee. Even though Sanders is giving Clinton a run for her money in Iowa and practically has home-state advantage in the next contest, New Hampshire, he has struggled to connect with blacks and Hispanics, who are big in the third and fourth early-voting states, South Carolina and Nevada.
Will his standard pitch that the billionaire class is taking over America be enough? So far it's worked out pretty well for him.
3. Benghazi and the '13 Hours' movie
It's back. Director Michael Bay's movie about the 2012 terrorist attacks in Benghazi, Libya, that killed four Americans, including a U.S. ambassador, came out in theaters on Friday. The movie is based on a book written in collaboration by surviving members of the CIA's security team, and already there's no shortage of controversy about what it portrays.
In an interview with The Post's Adam Goldman and Greg Miller, the former CIA officer in charge of Benghazi that night disputes the movie's storyline, saying one of the most pivotal moments in the film -- a CIA order to “stand down” rather than rush off to rescue U.S. diplomats a mile away -- never happened.
Whether fact or fiction, the action-packed drama actually avoids politicizing the incident, writes Post reviewer Michael O'Sullivan.
But Clinton, who was secretary of state during the time and whose response afterward has consistently been called into questions by conservatives, is being done no favors by having a re-telling of that deadly night brought to life for Americans less than three weeks for the first votes in the race are cast. Even if you don't think Clinton did anything wrong with regard to Benghazi, it's clearly not helpful to have it front-and-center again.
4. National security
This is Clinton's strong suit, and it's a topic she's dominated in the past three debates.
As she balances how to come across as hawkish as possible with a base that is increasingly worried about terrorism but also wary of military intervention in the Middle East, Clinton has not recommended undoing or changing in a major way any of President Obama's efforts to prevent the flow of would-be fighters to the Islamic State.
Sanders has struggled to match Clinton's intensity on national security and terrorism, preferring to talk about his bread-and-butter issue of economic inequality.
But that doesn't mean Clinton is immune to attacks from her challengers. Sanders tried to get a leg up in December's ABC debate by questioning Clinton's decision to support regime change in the Middle East while she was secretary of state.
5. Islamic State and terrorism
When Democrats gathered for their November debate, they were one day removed from the Paris attacks. The candidates got the chance to offer their solutions for how to counter the rising threat of the Islamic State and, most notably, Clinton not-so-subtly rebuked Obama's comments that the Islamic State had been "contained."
When they debated again in December, the Islamic State-linked mass shooting at a holiday office party in San Bernardino, Calif., was still part of the national conversation.
Each time, Clinton has tried to pivot to general election mode, going out of her way to criticize GOP candidates' prescriptions for rising fears of terrorism.
She has had harsh words for Sen. Ted Cruz's (R-Tex.) suggestion to "carpet bomb" the Islamic State in Iraq and Syria and Trump's proposal to temporarily ban Muslim immigrants from the country.
"Bluster and bigotry are not credentials" to be commander in chief, she said in a speech Tuesday at the University of Minnesota.
But the GOP's tough talk makes political sense in light of Obama's approval ratings on terrorism and the Islamic State, which are at at record lows, according to Washington Post-ABC News poll taken after the Paris attacks. Clinton is doing her best to draw a contrast with her former boss, but in a plan she outlined in November, she offered few major changes on what to do militarily in Iraq and Syria.
6. Gun control
Polls consistently show that as many as 90 percent of Americans support at least expanding background checks, but a majority of Americans are doubtful that changing gun laws will help prevent gun violence.
In other words, it's complicated.
Clinton has staked out a position on gun control to the left of her 2008 presidential campaign; she recently called for a renewal of the federal ban on so-called assault weapons.
Former Maryland governor Martin O'Malley has one of the strictest gun control platforms in the race: He wants to ban assault weapons and require that every person who buys a gun acquire a license and get fingerprinted.
Addressing gun control proved to be a struggle for Sanders in the first debate, as Clinton attacked him for a voting record she said looks more like and NRA supporter. (Sanders says he was representing his rural, Vermont community, which remains very pro-gun rights.) He specifically came under fire for voting for a 2005 law that gives gun manufacturers legal immunity from damages their guns might cause.
On Saturday night, Sanders said he would be open to a bill introduced in the Senate to reverse some key parts of the law. "As I have said for many months now, we need to look at the underlying law and tighten it up," Sanders said in a statement.
But how much the candidates can do on gun safety is an open question. Congress has failed to pass any gun control reforms. A proposal to ban terrorism suspects on a secretive no-fly list from being able to buy guns -- and one to expand background checks -- failed in the Senate one day after the San Bernardino shooting.
7. Climate change and energy
Democratic presidential candidates agree the government needs to step in to help stop climate change and invest in more clean energy, and all three candidates cheered a historic, 196-country agreement reached in Paris to try to limit the Earth's warming by 2 degrees Celsius above pre-industrial levels.
The litmus test among environmentalists has long been whether or not candidates would approve the fourth stage of the Keystone pipeline to ship Canadian oil to Nebraska, a politically touchy point for Clinton who recently said she opposes the pipeline but not before wavering for months.
Obama took that test off the table in November when he announced he wouldn't approve the pipeline.
Clinton has now shifted her focus to trying to invest in coal communities hit by a shift to clean energy: Before Democrats' November debate, she unveiled a $30 billion infrastructure and tax-break plan to do just that.
Sanders and O'Malley regularly talk about how climate change is one of America's most pressing problems as well.
8. Clinton's ties to Wall Street
On Sunday, The Boston Globe published a story saying that while a U.S. senator from New York, Clinton rarely signed onto bills related to the financial services industry -- regardless of whether banks in her district and those donating to her campaign supported or opposed the legislation.
Sanders immediately pivoted off that story to criticize his opponent as too weak on Wall Street. Breaking up the big banks, taxing Wall Street and adding more regulations to the financial world forms the nucleus of Sanders's populist campaign.
"Cutting it out is not good enough," Sanders said Sunday on CBS's "Face The Nation," making a reference to when Clinton in 2007 told big banks to "cut it out."
Clinton, meanwhile, is trying to distance herself from Wall Street -- saying Sunday she took more contributions from teachers than bankers.