Bernie Sanders has emerged as a serious threat to Hillary Clinton — but not because he's going to beat her.

Instead, if Sanders holds on to his recent second-place status in primary polls, and retains enthusiasm from the activist left, he'll ensure that a troublesome issue for Clinton — money in politics — will remain at the forefront of the Democratic campaign for months to come.

It's clear that Clinton has already been damaged by coverage of her family foundation's fundraising, with adults saying they disapprove of her handling of these questions by a 17-point margin in one recent poll. Beyond that, the Clinton team's eagerness to raise huge sums from the rich, and her own acceptance of hefty speaking fees from banks and corporations, has fed a perception that she's too close to the rich.

So Sanders, with his decades of activism against the influence of the wealthy, presents an appealing contrast for many progressives — one that seems tailor-made to sap enthusiasm for the frontrunner, in a way that a head-to-head race between Clinton and Martin O'Malley wouldn't.

Sanders's run shines a spotlight on the influence of the rich

While other candidates might opportunistically criticize Clinton on money in politics, Sanders is a uniquely credible messenger on the issue — he's been obsessed with checking the power of the wealthy since the 1960s. With inequality soaring and court decisions like Citizens United allowing more and more money to pour into our political system, he's lately been more vehement than ever that a "political revolution" is necessary.

Now his campaign is overwhelmingly focused on challenging the power of corporations and the super-rich. He wants the Democratic Party to wage a rhetorical war on the billionaire class, to better mobilize the general public against them, and break their power.

Clinton has a very different approach — she wants to win support from, and work closely with, many of the wealthiest people in the country. In contrast to Sanders, she's accepted millions in speaking fees and donations from corporations and banks for herself, her campaigns, and her family's foundation. And recent polls show her image has been hurt by all this.

Indeed, the New York Times's Eric Lichtblau and Nick Confessore reported recently that Clinton's allies are searching for "a new class of at least 20 Democratic donors who can give $5 million or even $10 million each" to Super PACs. One reason for this, of course, is that Clinton — like Obama and practically every previous major party presidential nominee in the modern era — thinks massive fundraising is necessary to win. Sanders, though, has denounced Super PACs.

In a race where Sanders is Clinton's main opponent, the potentially unflattering contrast between them on these issues will be spotlighted again and again. In every debate, Sanders will argue that the party needs to do more to soak the rich. If Clinton tries to argue, for instance, that Sanders's support for single-payer health care is unfeasible, some of his supporters may believe she's just carrying water for monied interests. The narrative that Clinton is tainted on money-in-politics issues could harden, leading to progressives losing enthusiasm for her candidacy and working less hard for her in the general election.

Clinton surely sees this threat coming — which could help explain why she's been so outspoken on campaign finance reform lately. She's suggested a constitutional amendment to get big money out of politics, and even told fundraisers that she'd only nominate Supreme Court justices who wanted to overturn Citizens United, the Washington Post's Matea Gold and Anne Gearan reported.

And it's worth remembering that Sanders is, in the end, a team player. Though he's critical of the Democratic Party, he clearly views Republican control of the White House as a much more serious threat. So if Clinton does defeat him to win the Democratic nomination, he'll likely endorse her. (Last September, he pledged that he'd never run as a spoiler third-party candidate if that could lead to the election of a Republican president, saying, "We've made that mistake in the past.")

Beyond that, a major question for 2016 is whether Clinton can channel the same enthusiasm among Democratic base voters that Obama did. If she spends the next seven months or so being on the defensive about big money — or trying to convince voters that Sanders's agenda is too far left — that task will grow even more difficult.

Yet Sanders's rise makes Clinton even more likely to win the nomination

Still, Sanders's emergence is great news for Clinton for one important reason — he's not a serious threat to beat her.

As Brian Beutler argues, it's important not to overstate the Sanders surge. Clinton has a massive lead in national and early state polls. She's locked up the vast bulk of establishment support. And many of the elements of her 2008 defeat don't apply to this race. Overall, she currently faces no serious threat from anyone for the nomination.

But the idea of Sanders winning the nomination seems particularly implausible. Remember that although Barack Obama was genuinely an underdog when he defeated Hillary Clinton in 2008, he wasn't considered an extreme and unelectable candidate by party elites. Though Clinton won most establishment support, Obama seemed to be a perfectly acceptable nominee, too — and once he won the upper hand in enough electoral contests, the party was happy to fall behind him.

This wouldn't be true for Sanders, who, of course, isn't even a member of the Democratic Party. He lacks the ties with the party establishment necessary to unify elites around him. Plus there will be serious concerns about his electability, considering he's a self-described socialist and all. So, however contentious the campaign with Sanders may get, Clinton's campaign will remain confident that she's not at risk of losing.

The upshot of this is that the enthusiasm among progressives for Sanders could crowd out other candidates with more potential appeal to the party's mainstream from catching on.

Take Martin O'Malley. A former governor of Maryland with a liberal record who's well-connected in his party, he has a profile much more like those who've won previous Democratic nominations than Sanders does. Right now, though, he's getting support from less than 3 percent of Democrats in both nationwide and early state polls.

So to make himself better known, O'Malley is trying hard to position himself as the progressive alternative to Clinton, including by proposing major, Elizabeth Warren-style financial reforms, and coming out strongly against the Trans-Pacific Partnership.

Yet it's not O'Malley but rather Sanders's rhetoric criticizing the power of the wealthy and his sweeping policy agenda that are impressing liberals and going viral among online activists.

If someone like O'Malley won Iowa or New Hampshire, Clinton's inevitability would flash before her eyes just like it did in 2008. But if Sanders managed to pick off an early state, it would mainly be an embarrassment for Clinton — not an existential threat.

So the Sanders surge — if it lasts — will be a double-edged sword for Clinton. It makes her already very strong position in the primary even stronger. But by sapping progressive enthusiasm for Clinton and spotlighting issues where she's weak, it could come at a cost.