Hillary Clinton and Senator Bernie Sanders have sparred over US involvement in the Middle East, gun control and economic policy in the first Democratic presidential debate, outlining competing visions for a party seeking to keep the White House for a third straight term.

Yet in a moment of political unity - and levity - Sanders leapt to Clinton's defence on the issue of her controversial email practises as secretary of state.

"The American people are sick and tired of hearing about your damn emails," Sanders exclaimed as the crowd in Las Vegas roared with applause on Tuesday night.

On the US campaign trail with the Democrat 'socialist'

A smiling Clinton reached over to shake his hand and said, "Thank you, Bernie."

While the five candidates onstage took issue with each other, they also repeatedly sounded traditional Democratic themes - such as fighting income inequality - that are sure to carry over to the general election campaign against the Republicans.

Al Jazeera's Patty Culhane, reporting from the debate in Las Vegas, said both Clinton and Sanders impressed during the debate, with the former first lady showing the polish she gained during the 2008 campaign.

She said that a significant policy difference between the two was on how to handle the spiralling crisis in Syria - with Clinton saying she would impose a no-fly zone over the country.

Clinton and Sanders also engaged somewhat heatedly over gun control, with Clinton saying that the Vermont senator had not been tough enough on the issue.

Sanders defended his gun control record, and called for better mental health services, stricter background checks on gun purchasers and closing the loophole that exempts gun shows from background checks

"It is clear that the candidates are going to attack each other - ever so politely," Culhane said, noting the difference in tone between the Democrats and Republicans during their respective debates.

Until now, Clinton and Sanders - who has emerged as her toughest competition - have circled each other cautiously and avoided personal attacks.

After Sanders, a self-described democratic socialist, derided "a casino capitalist process by which so few have so much," Clinton said it would be a "big mistake" for the US to turn its back on the system that built the American middle class.

The two also tangled over foreign policy, an issue where Clinton is often more hawkish than others in the Democratic Party.

Also hanging over the debate: the lengthy deliberations of Vice President Joe Biden, who is weighing a late entry into the Democratic race. Debate host CNN kept an extra podium on standby just in case Biden decided to show up. 

Joining Clinton and Sanders on stage in Las Vegas were a trio of low-polling candidates looking for a breakthrough moment: former Maryland Governor. Martin O'Malley; Jim Webb, a former Navy secretary and US senator from Virginia, and former governor and senator Lincoln Chafee, the Republican-turned independent-turned Democrat from Rhode Island.

Source: Al Jazeera And AP