– A no-fly zone and regime change became their biggest points of contention.

On a night that began with Bernie Sanders apologizing for his campaign aides improperly accessing rival presidential candidate Hillary Clinton's voter database, it was not political shenanigans but pronounced foreign-policy differences between the two leading Democratic candidates that proved to be a key fault line in the third Democratic debate Saturday in Manchester, N.H.

It is an issue that also created strange political bedfellows, with the Democratic candidates who disagreed with each other finding kindred spirits among the Republican presidential contenders.

At the ABC News debate, Democratic front-runner Clinton called for a no-fly zone over Syria. "One of the reasons why I have advocated for a no-fly zone is in order to create those safe refuges within Syria, to try to protect people on the ground both from Assad's forces, who are continuing to drop barrel bombs, and from ISIS," she said, calling for a diplomatic effort toward that end. "A no-fly zone would prevent the outflow of refugees and give us a chance to have some safe spaces."

Clinton's rationale for a no-fly zone — to support the rebels battling the Assad regime and protect refugees — is similar to that of Republican presidential candidate Marco Rubio. In an Aug. 25 op-ed for Foreign Policy, the U.S. senator from Florida offered his rationale: "The United States should work with our allies, both Arab and European, to impose a no-fly zone over parts of Syria that will prevent Assad's air force from dropping barrel bombs and chlorine gas bombs on civilian neighborhoods."

Both Clinton and Rubio believe the U.S. should go after Assad as well as the Islamic State in Iraq and the Levant (ISIL) that has taken over parts of Syria. "If the United States does not lead, there is not another leader," Clinton said Saturday. "There is a vacuum."

Rubio has used uncannily similar rhetoric: "When America doesn't lead, what's left behind is a vacuum, and the vacuum leads to chaos."

Clinton's closest Democratic rival, Sanders, opposes a no-fly zone and argued in the debate that the U.S. should steer clear of toppling dictators and avoid using military power to promote democracy abroad.

"Our differences are fairly deep on this issue. We disagreed on the war in Iraq," Sanders said, adding that he worries Clinton "is too much into regime change and a little bit too aggressive without knowing what the unintended consequences might be.

"The truth is it is relatively easy for a powerful nation like America to overthrow a dictator. But it is very hard to predict the unintended consequences and the turmoil and the instability that follows after you overthrow that dictator," said Sanders, a U.S. senator from Vermont. "So I think Secretary Clinton and I have a fundamental disagreement. I'm not quite the fan of regime change that I believe she is."

Underdog Martin O'Malley, lagging a distant third in the polls, sided with Sanders and said the U.S. shouldn't "let our lust for regime toppling get ahead of the practical considerations for stability in that region." He also opposes a no-fly zone over Syria.

The exchange revealed a foreign-policy rift that cuts across the Democratic and Republican fields and unites candidates who otherwise have little in common. Among Republicans, Clinton's support for a no-fly zone is shared by Rubio, Donald Trump, Jeb Bush and Chris Christie.

Sanders' and O'Malley's opposition to a no-fly zone is shared by Ted Cruz and Rand Paul — along with President Obama. While proponents of a no-fly zone believe it will help domestic rebel groups in their fight to topple Assad and defeat ISIL, opponents say such a move could backfire and lead to unpredictable consequences such as emboldening terrorist groups.