Tensions over gender and racial politics, foreign policy and electability set the stage for Tuesday night's Democratic presidential debate in Iowa, the last before the Feb. 3 caucuses.Why it matters: Three weeks out, polling gives Bernie Sanders momentum, but shows Joe Biden, Sanders, Elizabeth Warren and Pete Buttigieg within striking distance of each other in the first contest. Biden still leads nationally.The state of play: Sens. Sanders and Warren, rivals for the progressive mantle, broke a pact not to go negative on one other, with Warren's allegation this week that Sanders told her in 2018 he didn't think voters would pick a female nominee in the 2020 election.But campaign aides also tell me rival Democrats are poised to put Biden on defense over his foreign policy record, including support for the war in Iraq.Sen. Amy Klobuchar and billionaire Tom Steyer will also share the stage.There's no more time to waste in making a compelling closing argument to Iowans in particular, so the gloves are off.With Andrew Yang, Tulsi Gabbard and Deval Patrick falling short of the qualifying thresholds, this is the first time this cycle that not a single person of color will be on the debate stage — even after Democrats started the primary with the most diverse field in history.Michael Bloomberg also has not met the threshold to qualify — that's allowing the self-funding billionaire to continue ramping up what is expected to be a $1 billion-plus campaign without having to answer questions from rivals on the stage.The big picture: Candidates are preparing to take on Biden on foreign policy, in part because of the recent conflict between the U.S. and Iran, using that as an opportunity to question his judgement on these issues if elected president.Biden has relied heavily on foreign policy (with the help of former Sec. of State John Kerry) to make his electability pitch to voters throughout Iowa. A November CNN poll showed 48% of Democrats and Democratic-leaning independents think Biden is the best candidate to handle foreign policy, while 14% of those voters said the same of Sanders. A new Morning Consult/Politico poll suggests that gap is shrinking .Tuesday morning, Sanders tweeted a video from his campaign comparing his record to Biden's on the Iraq War (Biden voted for it; Sanders didn't); trade agreements like NAFTA; social security and Medicare; and student debt.Nina Turner, co-chair of Sanders' campaign, wrote a scathing op-ed last Sunday arguing that Biden has "repeatedly" disappointed black voters.Just a few weeks ago, Buttigieg called Biden's vote for the Iraq War the "worst foreign policy decision of my lifetime."Biden isn't normally the one to provoke an attack against a fellow Democrat on stage or off. Instead, he's tried to focus his ire on President Trump, which he did in a video released on Tuesday.A Biden advisor told Axios: "Some opponents seem to be recycling the same retired attacks we’ve seen throughout the race. They are not new, they haven’t worked in months, and they’ve backfired on people who’ve used them in the debates before."On Monday, Warren released a statement confirming reports that Sanders told her during a December 2018 meeting that a woman could not win the election in 2020.That came after a report released earlier this week revealed Sanders’ campaign volunteers were given a script that included sharp criticisms of Warren as a candidate.The moderators will more than likely ask both of them about this exchange and the brewing tension between them, but there's very little for either to gain by going negative against each other when their ultimate goal — and the goal of liberal party leaders — is to instead take on the more corporate candidates.Go deeper: Sanders and Warren split with three weeks until Iowa caucuses