Democratic debate as it happened: Clinton, Sanders and O'Malley face off before primary voting begins

And that's a wrap for the last Democratic debate before primary voting begins in February. Republicans will squeeze one last debate before the Iowa caucus on 1 February.

Sanders returns to one of his most popular topics: campaign finance reform. Calls for an end to super PACs.

Clinton, meanwhile, focuses on the Flint water crisis. She says the governor of Michigan failed the people of Flint, adding, "I want to be a president that takes care of the big problems."

O'Malley is asked if there is anything he wants to say that he didn't get to say. The question receives laughs from the crowd, particularly since O'Malley has barely been given time to speak. O'Malley mentions immigration, Puerto Rico and conflict in Central America.

Clinton is asked about her husband, former President Bill Clinton: "I am going to ask for his ideas. I'm going to ask for his advice."

Can I get 30 seconds, too?

- Martin O'Malley after Clinton is given extra 30 seconds after she's cut off by a commercial break

Next YouTube question comes from Marques Brownlee for O'Malley on privacy, technology and encryption. O'Malley responds: "The government should get a warrant, whether they want to come in via your back door or front door."

Clinton on her relationship to Russian President Vladimir Putin: "It's interesting."

On Syria:

• Sanders: 1st priority should be getting rid of Islamic State (Isis/Daesh) and 2nd priority should be to depose of Syrian President Bashar al-Assad
• O'Malley: We need more human intelligence on the situation in Syria and what would happen if Assad is deposed.

The debate quickly turns to Syria and whether the candidates would put "boots on the ground". Sanders points to the war in Iraq, while O'Malley thanks his fellow candidates for not referring to US servicemen and women as "boots on the ground".

The issue many were expecting: the Iran deal.

Sanders says that relations between the US and Iran should be normalised. The senator adds that the Iran deal is a positive step, but that he does not think the US should open an embassy in Tehran.

Clinton talks about her role in the sanctions initially placed on Iran and says she supports the Iran deal. However, she says that the US should continue watching Iran.

Candidates are asked moving away from fossil fuels and climate change. Sanders calls out Republicans for not believing in climate change, while O'Malley asks his fellow Democratic candidates to join him in pushing for 100% clean energy by 2050.

While Sanders wants to raise taxes to pay for free college and other entitlements, Clinton says she does not want to raise the taxes of the middle class.

Learn exactly where each candidate stands on regulating Wall Street (based on their campaign websites):

• Hillary Clinton
• Bernie Sanders
• Martin O'Malley

Clinton takes on Sanders and his alleged criticisms of President Obama. She says she agrees with Sanders on a number of issues, but says she will continue to support Dodd-Frank.

The debate now takes on the economy. Sanders is asked about his campaign video on Wall Street posted last week.

Second YouTube question comes from Connor Frente and it's for all the candidates. How will they get more young people to vote for them.

Clinton focuses on rising student debt, more jobs and moving forward on women's rights and gay rights.

Another question for Sanders. This time he's asked about labeling himself a Democratic Socialist. He says that the Democratic Party needs to be changed.

The debate, nearly an hour in, has been focused on frontrunner Clinton and Sanders. Viewers have noticed that O'Malley has been left largely out of the conversation—including Republican presidential candidate Rand Paul, who's Tweeting the #DemDebate.

Clinton and Sanders spar over Sanders's healthcare propositions. Clinton claims his new plan will destroy Obamacare, adding, "I don't want to see us start over again. I want us to defend and build on the Affordable Care Act and improve it."

We have to move away from treating the use of drugs as a crime and instead, move it to where it belongs, as a health issue. We need to divert more people from the criminal justice system into drug court, into treatment and recovery.

- Hillary Clinton

When asked how authorities should deal with the rise of heroin addiction in the US, Clinton calls for all officers to be properly trained and equipped with Narcan to deal with overdoses.

Franchesca Ramsey asks Sanders about investigations into deaths while under police custody.

Sanders says that any death under police custody should trigger a federal investigation. He adds that police departments should be demilitarised and should reflect the communities they serve.

Sanders is asked about polling and whether he thinks he can win the nomination when Clinton beats him 2-to-1 among minorities. He says his campaign has "momentum" and notes he has risen in the polls and even surpassed Clinton in New Hampshire and Iowa.

O'Malley says he agrees with Clinton and Sanders, that they both have flipped their views on gun control. The former Maryland governor says he's the only person on stage to actually pass comprehensive gun control. He adds, "I've never met a self-respecting gun owner that needed an AR-15 to down a deer".

Sanders is questioned about flipping his vote on a recent gun control bill.

Question: During my first 100 days in office, my top three agendas will be:

• Sanders: healthcare, raise minimum wage to $15/hour, more jobs, tax
• Clinton: creating more jobs/raising minimum wage/equal pay for equal work, fixing healthcare and fixing division among the parties
• O'Malley: raising minimum wage, climate change, a new agenda for America's cities

Clinton is up first. The former secretary of state invokes civil rights leader Martin Luther King Jr and discusses the fight for higher wages.

Sanders, a former civil rights activist himself, also discusses MLK and minimum wage, while bringing up campaign spending.

O'Malley, like those before him, brings up MLK but also thanks South Carolina for showing that "love will have the final word" following the deadly shooting that left 9 churchgoers dead.

Candidates have 60 seconds to answer and 30 seconds for rebuttals. Each candidate has 30 seconds for opening statements.

As is customary the debate kicks off with a teaser of the three candidates as they take each other on the issues of gun control and healthcare.

We're just minutes away from the debate's start and the candidates have made their way on the stage. Prior to the debate, Sanders addressed demonstrators outside of the debate venue who are calling for a rise in the minimum wage.

While O'Malley has trailed behind his fellow candidates, Sanders has managed to surge ahead in the polls, giving Clinton a run for her money. A recent poll by Monmouth University found the Vermont senator had surged past the former secretary of state in New Hampshire (53% to 39%). A similar poll by Quinnipiac University found Sanders had edged out Clinton in Iowa (49% to 44%).

The fourth Democratic debate is set to start at 9pm EST/2am GMT featuring all three candidates. The latest poll by NBC News and the Wall Street Journal reveal the Democratic contenders stand as follows among the party's voters:

• Hillary Clinton: 59%
• Bernie Sanders: 34%
• Martin O'Malley: 2%