Why Bernie Sanders has the most to prove in Democratic presidential debate

Vermont Sen. Bernie Sanders faces the biggest test yet of his insurgent presidential campaign on Saturday night, when he faces off with Democratic front-runner Hillary Rodham Clinton in the party's second primary debate.

His goal is clear: Reset a contest that increasingly looks like little more than a march to the nomination for Clinton.

That effort will be complicated by fresh terrorist strikes that have captured the world's attention. Despite Sanders' focus on domestic issues, national security and foreign policy will play prominent roles in the debate, with the string of deadly attacks in Paris that killed more than 120 people front and center.

All the candidates quickly denounced the attacks in statements on Friday night. Party officials said the forum will continue as planned.

Foreign relations is an area where Clinton, a former secretary of state, is in the strongest position to talk about the attacks and the U.S. effort to dismantle the Islamic State group. But her tenure is tied to that of Obama, who's struggled to contain the threat from Islamic militants in Syria and associated terror attacks across the globe.

A spate of good news for Clinton since the party's first debate a month ago has helped her rebuild a lead in the early voting states, an uptick that comes amid other signs the party is coalescing behind her.

An Associated Press survey of superdelegates published Friday found that half of the Democratic insiders are publicly backing Clinton.

Sanders may have inadvertently facilitated some of her progress in the first debate, when he seemed to dismiss the controversy over her use of a private email account and server by saying Americans are tired of hearing about her "damn emails."

Since then, he's given her no more passes.

Though careful never to mention Clinton by name, Sanders has drawn a series of contrasts with the former secretary of state on issues that include her backing of the war in Iraq, trade and the minimum wage.

Sanders' advisers say he plans to discuss the email issue only if the moderators of the debate in Des Moines, Iowa, bring it up. That could be a signal to organizers that he's is open to the topic.

"He's definitely going to cut a harder contrast on core issues," said Larry Cohen, a senior adviser to Sanders. "But it's not going to be over personal style."

The problem for Sanders is that Clinton agrees with him on some of the core domestic issues of his campaign, having shifted to the left in recent weeks to oppose construction of the Keystone XL pipeline and the Trans-Pacific Partnership trade deal.

"It's really tough for him," said Gina Glantz, manager of Bill Bradley's 2000 presidential campaign, which posed a primary challenge to then-Vice President Al Gore. "He's in a difficult position where his current arguments aren't enough to get beyond his core voter."

While Sanders aides bragged about their candidate's lax preparation for the last debate, they shuttled him to his campaign headquarters in Burlington, Vermont, for mock sessions before this match-up. Clinton, too, has kept her schedule relatively clear over the last several days, leaving plenty of time for rehearsals.

"They are absolutely prepared for the fact that Bernie's going to come out swinging," said Maria Cardona, a Democraticstrategist who worked for Clinton's failed 2008 White House campaign. "The question is how it's going to happen."

Clinton supporters say their candidate will remain focused on laying out her vision for the future rather than striking back at Sanders. Her campaign has about $15.2 million in television advertising planned through mid-February, compared with a $3.2 million Sanders ad buy that ends next week, according to Kantar Media's CMAG advertising tracker.

The Service Employees International Union, an influential force in Democratic politics, is expected to issue their endorsement on Tuesday, according to people knowledgeable about the union's process. Clinton has been backed by more than 72 percent of members in all their internal polling, including the most recent survey conducted a few weeks ago.

Her team is hoping to notch another win after a series of strong moments since the first debate. Clinton has benefited from Vice President Joe Biden's decision to forgo a run and well-received testimony before a Republican-led congressional panel investigating the deadly 2012 attacks in Benghazi, Libya.

They're also trying not to alienate the Sanders backers whose support they'll need should Clinton win the nomination.

"As a front-runner your job is to do no harm," said Cardona. "She's going to want to be a comfortable home for the Bernie supporters toward the end of this process."

Sanders, too, may face tougher attacks. Former Maryland Gov. Martin O'Malley, who's struggled to break 5 percent in national preference polls, has questioned Sanders' commitment to the Democratic Party and President Barack Obama, still a popular figure among Democrats.

A more aggressive tone would mark a shift for a race that has so far been notable for its civility. Democrats have spent months boasting about the substantive tone of their contest, attempting to set-up a favorable early contrast with the often carnival-like insults of the crowded Republican primary.

Their bragging may come to an end after Saturday night.