Donald Trump's rise to the top of the Republican polls has been incredible. But it's obscuring a story that might ultimately prove more meaningful: Bernie Sanders's increasingly serious challenge to Hillary Clinton.

New polls show Sanders leading Hillary Clinton in both Iowa and New Hampshire. His leads aren't Trump-size — at least not yet — but they were secured without the wall-to-wall media coverage that attends Trump, without the name recognition Trump brought to the race, and against a much stronger frontrunner than Trump faced.

And Sanders has built those leads while remaining, well, Sanders. He promised he wouldn't run a negative campaign, and he hasn't — a fact that Clinton allies privately mention with relief. He hasn't signed on with a Super PAC or begun taking money from the kinds of donors he campaigns against. His campaign has been free of stunts and provocations and dense with policy proposals and issue papers. He's attracting supporters the old-fashioned way — by convincing people he's the kind of politician they want to back.

There is nothing inevitable about any of this. It was not obvious six months ago that Sanders would pull ahead of Clinton in Iowa and New Hampshire. Nor can his rise be explained away as simple "anyone but Hillary" sentiment: Sanders holds huge leads over Vice President Joe Biden, former Govs. Martin O'Malley and Lincoln Chafee, and ex-Sen. Jim Webb.

In a normal year, Sanders's rise would be a shocking, unbelievable story that would be dominating political media — it's just currently being overshadowed by Trump's story, which is arguably more unbelievable, and certainly makes for better TV.

The anti-Trump

There have been attempts to paint Bernie Sanders as the Democratic Party's Donald Trump. But the truth is that Sanders is more like the anti-Trump.

Where Trump has never held elected office, Sanders is one of the longest-serving members of Congress; where Trump calls himself a Republican but seems to loathe his party, Sanders calls himself an independent socialist but acts like a loyal Democrat; where Trump delights in attacking his fellow candidates, Sanders refuses to go negative; where Trump heightens the contrasts between him and his critics, Sanders has been unveiling new policies to quell doubts from Black Lives Matter activists; where Trump is limiting the Republican Party's ability to reach beyond its base and win minority voters, Sanders is trying to expand the Democratic Party's base among the white working class and evangelicals; where Trump is a billionaire attempting to take over American politics, Sanders is a congressman of unusually modest means trying to stop billionaires from taking over American politics.

And that's why Sanders's rise might end up being more influential in the Democratic Party than Trump's rise will be in the Republican Party. Trump is functionally at war with the GOP: His policies, where they exist, are often at odds with party orthodoxy, and neither he nor his supporters have any kind of hold on the Republican Party's infrastructure. The Republican Party doesn't want to co-opt Trump so much as it wants to destroy him and scrub his virus from its bloodstream.

By contrast, Sanders, who is already the ranking member on the Senate Budget Committee, is pushing the Democratic Party in a direction many of its most influential members — from officeholders to labor unions — already want it to go. He's showing that the political style initially associated with Elizabeth Warren wasn't dependent on her; that there's a real constituency in the Democratic Party, and perhaps even beyond it, for politicians who fight economic inequality by fighting political inequality. In some ways, Sanders is a better test case for that proposition than Warren would have been, as, unlike Warren, he wasn't considered a wildly charismatic politician before this campaign, so his success makes a stronger argument that it's the message that has resonance, not just the messenger.

To put it simply, it's very unlikely that Trump is going to persuade the Republican Party that the proper position on immigration is to implement mass deportations and make Mexico pay to build a wall. But it's entirely possible Sanders could convince Democratic Party leaders that campaign finance reform is a much more important issue than they had previously recognized, and that a serious, root-and-branch overhaul of the system should be the party's top priority.

Sanders is still a long shot. He trails Clinton badly in national polls and in more diverse states. But if his rise continues, it may prove more durable, and more significant, than Trump's political stardom.

Watch: Ezra Klein's interview with Bernie Sanders