The long-simmering rivalry between former Vice President Joe Biden Joe BidenThe Memo: Warning signs flash for Trump on debates Senate Republicans signal openness to working with Biden National postal mail handlers union endorses Biden MORE and Sen. Bernie Sanders Bernie SandersSenate Republicans signal openness to working with Biden Hillicon Valley: DOJ indicts Chinese, Malaysian hackers accused of targeting over 100 organizations | GOP senators raise concerns over Oracle-TikTok deal | QAnon awareness jumps in new poll Schumer, Sanders call for Senate panel to address election security MORE (I-Vt.) is coming to a head.

Sanders has gone on the attack with the Iowa caucuses less than a month away, taking aim at Biden’s main calling card of electability by arguing the former vice president’s record shows he’s not the strongest Democrat to nominate against President Trump Donald John TrumpBarr criticizes DOJ in speech declaring all agency power 'is invested in the attorney general' Military leaders asked about using heat ray on protesters outside White House: report Powell warns failure to reach COVID-19 deal could 'scar and damage' economy MORE.

“Joe Biden is a personal friend of mine, so I’m not here to, you know, to attack him,” Sanders told the Los Angeles Times’s editorial board in an interview late last month.

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“But my God, if you are, if you’re a Donald Trump and you got Biden having voted for the war in Iraq, Biden having voted for these terrible, in my view, trade agreements, Biden having voted for the bankruptcy bill. Trump will eat his lunch.”

Trump’s decision to authorize an airstrike that killed the leader of Iran’s elite Quds Force has put foreign policy on the front burner of the campaign and brought new attention to Iraq, where political leaders are calling for the removal of U.S. troops.

Sanders has jumped on the escalating tensions to pitch his anti-interventionist vision for U.S. foreign policy and remind voters of his 2002 vote against the authorization of the use of military force in Iraq. Biden, he’s quick to note, voted in favor of that measure.

“Joe Biden voted and helped lead the effort for the war in Iraq, the most dangerous foreign policy blunder in the modern history of this country,” Sanders said in an interview with CNN’s Anderson Cooper on Monday.

The attacks from Sanders come as polls suggest he and Biden are both in contention to win Iowa’s caucuses and New Hampshire’s primary.

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A CBS News–YouGov poll of likely Iowa Democratic caucusgoers released on Sunday showed the two candidates knotted in first place at 23 percent, along with former South Bend, Ind., Mayor Pete Buttigieg Pete ButtigiegBogeymen of the far left deserve a place in any Biden administration Overnight Defense: Woodward book causes new firestorm | Book says Trump lashed out at generals, told Woodward about secret weapons system | US withdrawing thousands of troops from Iraq A socially and environmentally just way to fight climate change MORE (D).

Another Iowa poll from Emerson College released last month showed Biden and Sanders in a statistical tie, with 23 percent and 22 percent support, respectively.

Public polling in New Hampshire, which holds its primary on Feb. 11, shows a similar rivalry between the two. A CBS News–YouGov survey in New Hampshire conducted in late December and early January showed Sanders narrowly leading Biden 27 percent to 25 percent, well within the poll’s margin of error.

Despite their policy differences, the two durable candidates are both targeting working-class voters, who have yet to settle on a single candidate.

To win the primary, the veteran politicians — Biden is 77 and Sanders is 78 — may need to go through each other.

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“They’re not fighting to eat into each other’s bases, but there are a lot of voters out there who don’t have a firm commitment to any candidate,” Brad Bannon, a Democratic strategist, said. “There’s still a lot of fluidity. They’re going after the undecided voters, your swing voters.”

Sanders’s pivot to Biden is not entirely unexpected. His allies have wanted him for months to take a more aggressive stance against the former vice president.

“When you’re in a multicandidate campaign, you have to go after the candidate who poses the biggest threat to you,” Bannon said.

The former vice president has been less direct in confronting Sanders, opting instead to focus on Trump in his campaign appearances.

Biden has sought to use the killing of Iranian Gen. Qassem Soleimani to talk up his foreign policy experience, first as a member of the Senate Foreign Relations Committee and later as vice president.

“It’s not to suggest I haven’t made mistakes in my career, but I would put my record against anyone in public life in terms of foreign policy,” Biden told supporters at a campaign rally in Des Moines, Iowa, on Saturday.

Still, he has taken implicit aim at progressives like Sanders and Sen. Elizabeth Warren Elizabeth WarrenNo new taxes for the ultra rich — fix bad tax policy instead Democrats back away from quick reversal of Trump tax cuts It's time for newspapers to stop endorsing presidential candidates MORE (D-Mass.), decrying their “my way or the highway” brand of politics and casting himself as the candidate best equipped to unite the country in the wake of Trump’s presidency.

That unifying theme was present on Tuesday as Biden delivered a statement on the escalating tensions between the U.S. and Iran, saying that Americans “need to use our system to bring us together as a nation — not abuse it to rip us apart.”

But he also worked in a rebuttal to his progressive rivals, who have criticized his calls for compromise as being callow or antiquated.

“That’s not a naive or outdated way of thinking,” he said, referring to a frequent point of criticism from progressives, including Sanders and Warren. “That’s the genius and timelessness of our democratic system, which has, for more than 240 years, allowed us to remake ourselves, reckon with our shortcomings, and move ever forward.”