That's when the two rivals line up on stage at the first Democratic debate of the 2016 campaign, sponsored by CNN and Facebook. The encounter will provide a crucial opportunity for Clinton and Sanders -- the leading Democratic contenders -- to contrast their personalities, experience and approach to the key issues in the campaign.
Though Clinton and Sanders have rarely mentioned each other's names, they are clearly reacting to each other and their rival's potential weaknesses. Sanders took aim at Clinton's Wall Street record and Iraq vote over the weekend; she put him on the defensive on guns and his poor standing with minority voters.
Until now, they have each had good reason for avoiding full contact with the other. Clinton hasn't wanted to elevate Sanders and his surprisingly strong poll numbers, while Sanders has wanted to maintain his untraditional, above-the-fray image.
On Tuesday, that calculus will change. And the distinctions they've subtly staked out on a range of issues are only likely to grow sharper.
Prepping positions for first debate
In the weeks leading up to the debate in Las Vegas, the two Democrats have been carefully finessing their political positions in relation to each other and their party's wide coalition, offering clues about how they will spar Tuesday night.
Sanders has been signaling he will try to strike a contrast with Clinton on reining in Wall Street and on her record of support for military interventions overseas. The former secretary of state, meanwhile, is under pressure to prove to progressives who have flocked to Sanders that she genuinely cares about the middle class. She's expected to highlight her differences with her rival on gun control and to demonstrate the broad support she has among minority voters -- a key sector of the Democratic coalition where Sanders is struggling.
As he limbered up for their clash, Sanders threw down the gauntlet on the Iraq War -- a thrust that Clinton has struggled to counter in the past -- hinting that she has hawkish views that are out of step with the majority of Democratic voters.
His campaign issued a statement reminding voters that he, then a member of the House of Representatives, voted against authorizing the Iraq war in late 2002. At the time he argued that the conflict would destabilize the Middle East, kill large numbers of Americans and Iraqi civilians and hamper the war on terror against al Qaeda.
The statement did not once mention Clinton -- but it did not have to. The then-New York senator did vote to authorize the Iraq war, and that vote was one of her greatest vulnerabilities in the 2008 Democratic campaign against Obama, who also opposed the war.
The Sanders statement raised the possibility that Clinton's vote could haunt her for a second presidential campaign.
"Democrats are no more fond of the Iraq war now than they were back then. That could be a problem," Peter Beinart, a foreign policy expert and CNN contributor, said Monday. He added that another Democratic candidate, former Virginia senator and Vietnam war veteran Jim Webb, who was also against the war, could double-team with Sanders to cause trouble for Clinton on the issue.
Sanders has also been staking out territory to Clinton's left on Syria. The former secretary of state recently distanced herself from Obama's much-criticized policy on the vicious civil war by calling for a no-fly zone to be set up to shield refugees.
Sanders issued a statement earlier this month pointing out that he opposes such an idea, warning that it could "get us more deeply involved in that horrible civil war and lead to a never ending entanglement in that region."
The statement appeared to be a clear appeal to Democrats who share Obama's antipathy toward getting the United States entangled in another Middle Eastern conflict and who are wary of Clinton's more activist instincts on foreign policy.
Attacks on Wall Street
Sanders is not alone in seeing Clinton's foreign policy record as a vulnerability. Another Democratic candidate, former Maryland Gov. Martin O'Malley, also picked up on her Syria position Sunday -- saying on CNN's "State of the Union" that a no-fly zone was not advisable and warning that the former secretary of state was "always quick for the military intervention," apparently referring to her previous support for military action in nations such as Iraq and Libya.
Another area where Sanders seems more in tune with the progressive Democratic base is on Wall Street, especially since he has raised most of his money from small donors -- unlike the former secretary of state, who has been relying on big budget fund-raising events with rich contributors. Even with his small-donor focus, Sanders is nearly neck-and-neck in the fund-raising race with Clinton.
Clinton has made strenuous attempts to connect with what her campaign has called "regular" Americans, stressing the need to raise up the middle class to feel the benefits of the economic recovery. But Sanders has said that she hasn't done enough, an argument he may expand upon on the debate stage.
"People will have to contrast my consistency and my willingness to stand up to Wall Street and corporations with the secretary," Sanders said on NBC's "Meet the Press" on Sunday.
The Vermont senator also will likely draw an implied contrast with Clinton on two other issues -- the Trans-Pacific Partnership trade pact the United States and 11 other nations reached this month and the Keystone XL pipeline designed to carry oil from Canada's tar sands across the United States.
Clinton now says she is a 'no' on both issues, but she took months to get there -- despite fervent opposition to both projects from the left flank of the Democratic Party.
"I am glad that she has reached that conclusion," said Sanders in Washington last week when asked about Clinton's opposition to a trade agreement she championed repeatedly as secretary of state. "This is a conclusion that I reached from day one."
Yet Sanders is not alone in curating the battlefield for the Democratic debate. Clinton, while getting in line with progressive Democratic positions on big issues, has also been preparing to strike key contrasts with Sanders.
Differences on gun control
Guns are one policy issue where Sanders is not completely in sync with the Democratic base, so Clinton is likely to exploit it on Tuesday night.
She has been promising a forthright effort to enact new gun control laws after a string of recent mass shootings. It partly seems to be an attempt to focus attention her rival's record on guns, which recently saw him express his openness to reforms that would hold gun manufacturers liable for crimes committed with their weapons.
Clinton has also spent the runup to the debate cementing her links to key voting blocs of the Democratic coalition -- especially in sectors of the party where Sanders is weak. She can point to broad appeal in the party, which could be key to eventually blunting the challenge from Sanders after early-voting contests in the less diverse states of Iowa and New Hampshire where he is strong.
In recent weeks, Clinton has met representatives of the Black Lives Matter movement and has even criticized Obama for not going far enough in changing immigration laws.
"Hillary has done a lot of work leading up to this debate that has pretty much gone unnoticed," Patti Solis Doyle, Clinton's 2008 campaign manager, said on CNN on Monday.
"She's rolled out Latinos for Hillary, she has rolled out Women for Hillary, she has met with the leadership of Black Lives Matter, she has checked a lot of boxes walking into this debate," she noted. "I think she is going to display tomorrow night (Tuesday) her vast support among this coalition."
The challenge that Sanders faces reaching out to minority voters, who are a vital part of the Democratic Party voting bloc, was underscored by a new CNN poll Monday finding that only 1% of nonwhite voters in the important early voting state of South Carolina favor him.
That is a showing that Sanders must improve on if he is to come from behind and beat Clinton for the nomination.