Vermont senator Bernie Sanders is a self-proclaimed socialist who wants to dismantle Wall Street, drastically scale back defense spending, and return power to the “people.” Nearly every one of his policy positions places him well to the left of his rival for the Democratic nomination, Hillary Clinton. So it stands to reason that Republican presidential candidates would be in a stronger position to defeat him come November, right?

Wrong. In fact, Sanders fares far better than Clinton does against every major GOP candidate — and the margin between the two Democratic competitors may well be growing.

Take Donald Trump. Clinton currently enjoys a 5-point polling advantage over the billionaire real estate mogul, 47-42 percent, according to polling averages compiled by Real Clear Politics. She has led him in nearly every poll since he entered the race – in some cases, by a double-digit margin. Clinton also enjoys a 3-point lead over former Florida governor Jeb Bush, 46-43 percent, and that lead has tended to hold steady, too.

But Sanders’ margin against both GOP leaders turns out to be much larger — a whopping 10 points, 50-40 percent, over Trump, and a robust 5 points over Bush, 47-42 percent. Compare Sanders’ appeal with other GOP candidates and the pattern is the same. For example, Clinton and Ted Cruz are in a dead heat, but Sanders bests Cruz by more than 3 points. In general, the Vermont senator does 3-5 points better against GOP opponents than the presumptive Democratic front-runner does.

How is this possible? The answer’s simple: it’s Sanders relative advantage with key voter groups, including independents, men, voters under 35, and those without college degrees. Sanders not only outpaces Clinton among these voter groups he does so by a surprisingly large margin.

Consider the breakdowns reported in the most recent poll conducted by Quinnipiac. According to Quinnipiac. Clinton leads Trump by 12 points among independents and by 38 points among voters under 35, two reasons she enjoys a strong overall advantage over the reality TV star. But among these same groups, Sanders leads by 26 points and 53 points, respectively – dwarfing Clinton’s numbers.

Gender is another area where Sanders outpaces Clinton – and not just with men. Clinton trails Trump by 9 points among men, but leads the GOP front-runner by 16 points along women. However, Sanders not only splits the male vote with Trump, he also leads by 19 points among women, three points higher than Clinton.

Amazingly, most of these Sanders voter group advantages hold firm no matter who the GOP candidate is. Take Marco Rubio. Florida’s junior senator has a 20 point lead over Clinton among men in head-to-head polling, compared to just a 4 point deficit among women. But against Sanders, Rubio leads by just 6 points against men, while trailing among women by 4. Rubio leads Clinton by 9 among independents, but Sanders leads Rubio by 3 among this same group – a 12 point swing. Results for Cruz and Bush follow a similar pattern.

Favorability is another metric that could give Sanders an important general election advantage. Clinton and Trump both have deeply negative net favorability ratings. Clinton is at -17 and Trump is at -25, and close to 95 percent of voters say they are set in their judgment. Sanders’s rating, by contrast, stands at +9 and nearly 20 percent of voters say they haven’t heard enough about him to decide one way or the other.

In fact, the evidence suggests that Sanders is increasing his lead over GOP candidates relative to Clinton. Last August, Quinnipiac conducted similar head-to-head match ups at a time when 40 percent of voters – twice the number, currently — said they had not heard enough about the Vermont senator to decide if they viewed him favorably. Back then, Clinton led Trump by 4 points in a head-to-head match-up while Sanders led Trump by just 3. Six months later, it’s obvious that Clinton has barely improved her advantage over Trump. By contrast, Sanders’ advantage has grown more than threefold.

For Clinton, there is a real — and growing — danger here. If Sanders continues to poll this favorably against GOP candidates, it will completely undermine one of her key arguments with Democratic voters – that she’s more “electable” than Sanders in the general election. That could allow Sanders to woo wavering Democratic voters who say they lean his way but who are worried that he may not be able to compete as effectively as Clinton with the GOP nominee in November.

For Republicans, Sanders’ polling challenge is good news up to a point. It allows the party to paint Clinton as a “weak” candidate – so weak that even her “socialist” rival bests her with key segments of the electorate. But this tactic could also backfire if the Vermont senator somehow manages—against all expectations – to capture the Democratic nomination himself. No longer just a damaging foil to Clinton, he would emerge as a formidable challenge in his own right – leaving the GOP, like her, scrambling for an effective counter-strategy.