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Why Bernie Sanders may be Joe Biden's best surrogate

Analysis: When the Vermont senator calls himself the race's true progressive, it pushes many voters whose top priority is beating Trump toward the former vice president.

Bernie Sanders is stacking the rest of his chips on the same risky bet that may have just collapsed his odds of winning the Democratic presidential nomination.

Sanders frames himself as the true progressive in the race and Joe Biden as too centrist for the Democratic primary electorate. On Friday, he contrasted their records on everything from trade policy to gay rights and abortion rights, emphasizing those differences enough to overshadow an underlying point that they would make Biden vulnerable to attacks from President Donald Trump.

"As we enter the moment in this campaign where we come down to a two-person race, I think it is important for us to differentiate our records, and I intend to do that," Sanders said.

But with each attack, Sanders may inadvertently be making the best case for Biden, who became the newly minted front-runner and prohibitive favorite in the span of four days in which he won South Carolina's Feb. 29 primary and most of the 14 states where voters went to the polls Tuesday.

Sanders is right that Biden less represents the progressive id of Democratic primary voters. And in an election year in which their party has demonstrated that it is motivated in part by fear — with many liberal primary voters feeling they should suppress their own progressive impulses to defeat Trump — highlighting Biden's centrism could be a fatal strategic miscalculation on Sanders' part.

In exit polling across 12 of the 14 Super Tuesday states, NBC News found that 63 percent of voters said they cared more about choosing a nominee who could beat Trump, while 34 percent said they preferred one who agreed with them on major issues.

Sanders also contends that he is electable, and polling suggests there's little difference between him and Biden in head-to-head matchups with Trump. He introduced his recounting of the contrasts between his record and Biden's Friday with the argument that Trump would exploit Biden's positions in a general election matchup.

"These are issues that Trump will certainly be emphasizing in the campaign," Sanders said.

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But in singling out policy matters such as federal funding for abortion rights, where Biden's past positions are more in line with Republican orthodoxy, Sanders demonstrated that contrasting his progressive credentials against Biden's is the turf he wants to fight on.

The data, and Biden's willingness to cede the title of progressive champion to Sanders, suggest that's a perilous choice.

Of the 63 percent who favored a nominee who could beat Trump, 37 percent said they voted for Biden and 24 percent said they voted for Sanders. At the time, several other candidates were still in the running or had exited the race so recently that they remained on the ballot. Of the 34 percent who wanted a nominee who shared their values, 42 percent favored Sanders and 25 percent went for Biden.

Simple arithmetic yields this conclusion: Twenty-three percent of all voters cared most about beating Trump and chose Biden, while 14 percent cared most about a candidate sharing their views and picked Sanders. The Vermont senator actually scored a higher share among voters who wanted a candidate who could beat Trump — 15 percent — than those who preferred someone who agreed with them on the issues.

Biden, keeping his focus firmly on the general election even when that hurt him in the first few states to vote, has shown little inclination to fight Sanders for the title of "most progressive candidate." That makes Sanders' decision to battle for territory he already owns all the more curious. But Sanders' critics have long contended that the only political math he understands is division.

Biden appears to have attracted support from the departures of other candidates who found Sanders' platform too progressive and his style too aggressive, including Amy Klobuchar, Pete Buttigieg and Michael Bloomberg, all of whom endorsed Biden this week.

The notion that Sanders is cementing a centrist reputation for Biden — which would help Biden compete with Trump for swing voters who also reject Sanders' brand of progressivism — is concerning enough to Republicans that they are moving quickly to combat it.

Americans for Tax Reform, the group founded by Republican anti-tax activist Grover Norquist, sent out a press release Friday with the large-font headline "Joe Biden is Not a 'Moderate.'" The release included links to clips of Biden comments about repealing Trump tax cuts, raising capital gains taxes and eliminating the use of fossil fuels.

That echoed the messaging of Trump's campaign "war room" online.

Ultimately, the challenge for Sanders is to convince Democratic voters that Biden won't be able to draw an effective contrast against Trump in the general election by convincing them that Biden doesn't agree with them on the issues. He's trying to do that at a time when most Democratic primary voters care more about picking a nominee who can defeat Trump than one who shares their views.

If Sanders is persuasive on the first part — showing that Biden is more centrist than the average Democratic primary voter — but not in making the case that defeating Trump requires nominating a candidate who is more in line with the party's progressive wing, he will have done Biden a major favor.

So far, it appears he's done just that.