Hillary Clinton's lead in the polls has largely evaporated, and young voters are the reason why.

In Quinnipiac's survey, her lead over Donald Trump among voters under 35 has fallen from 24 points in August to 5 points today; Fox News' survey showed a similar decline, from a 27-point lead to a 9-point one.

This is a very serious problem for a Democrat, because old people generally vote Republican. In 2008, Barack Obama won 66 percent of 18-29 voters, and 60 percent in 2012 — providing his margin of victory in the latter case. If Clinton cannot win young voters by a sizable margin, Trump could well take the presidency.

So Clinton is rolling out a pitch to millennials this week, trying a new message and deploying her most credible left-wing surrogates — Bernie Sanders, Michelle Obama, and Elizabeth Warren — to try and win back young voters. By emphasizing her reasonably good domestic policy, she ought to be able to win back enough support — particularly from libertarian Gary Johnson, who has picked up some 29 percent of the youth vote, according to a recent poll.

Older Clinton partisans have reacted with fury at news that Clinton's support for young people is crumbling (while sometimes ignoring the fact that their generation is actually going for Trump). Justification for such outrage aside, I suggest that scolding people is unlikely to work as a vote-getting strategy. The simple fact is that voting is an emotional as well as tactical decision, and references to vote-splitting and Ralph Nader are going to have limited purchase.

It's honestly rather mysterious why Clinton's support is plummeting among young voters. Kevin Drum suggests it's all Bernie Sanders' fault, but that doesn't square with a 20-point collapse just in the last few weeks. The primary is long since over; the Democratic convention where Sanders gave her a lavish endorsement was nearly 2 months ago.

Besides, contrary to Drum's carping about Sanders' supposed smears, the Democratic primary was unusually polite and substantive, as Matt Ygelsias writes. If that mild of an election did terrific damage to Clinton's popularity among young people, it must have been extraordinarily weak to begin with.

The truth is that Clinton has been weak with young people for years and years. Obama cleaned up young voters in 2008, winning them 60 percent to 35 percent. Sanders won them by something like 71 percent to 28 percent — and starting from a greater disadvantage than any primary challenger in a century at least.

There are several plausible reasons for this. The last decade and a half has featured an endless parade of disasters: the botched wars in Afghanistan and Iraq, the financial crisis, and the inadequate recovery. Clinton has been a high-profile member of the political elite for every one of those events, and so, only somewhat unfairly, gets tarred by association. In particular, Clinton's vote for Iraq is surely still her single biggest problem with young people.

But at any rate, what changed over the last month? Well, Clinton did spend much of August trying to win Republican votes, saying she wants to "be the president for all Americans. Democrats, Republicans independents," contrasting Trump unfavorably with Ronald Reagan, and winning the support of such conservative luminaries as Meg Whitman, in addition to neoconservatives like Max Boot and Robert Kagan she already had on board. In return those same Republicans are reportedly expecting jobs, access, and policy influence during a Clinton administration.

So if I were a very young left-wing Sanders Democrat trying to figure out who to support in November, I suspect this is the absolute last thing I would want to be hearing. It plays into every stereotype of Clinton as a conservative-lite foreign policy hawk. And speaking personally as a somewhat-disgrunted Sanders supporter, I don't want to hear Clinton rehabilitating the keystone architect of movement conservatism and wooing the people who gave us George W. Bush and the Iraq War. I want her to lash Trump to the GOP in the process of grinding it into paste up and down the ballot.

If she won't do that (which she won't), then at least Clinton can stop publicly pitching Republicans and start making the case for her domestic program, which really is reasonably good. Her plan for 12 weeks of paid leave, while inadequate, would be a godsend to any young family. Pressed by Sanders, she recently adopted a public option for ObamaCare, Medicare opt-in at 55, and debt-free college for people making less than $85,000 (gradually increasing to $125,000). She emphasized these at her campaign event in Philadelphia on Monday, which is a good start. With help from Sanders and Warren, she should sound reasonably credible on this front.

Her message could be further strengthened by looking at a notable absence from Clinton's campaign — namely, the constant bleating about balancing the budget that usually characterizes Democratic campaigns. As Mike Konzcal writes, Democrats have quietly shifted their economic policy orientation away from the austerity and neoliberalism of the old New Democrat days. The financial crisis and ensuing crummy recovery profoundly discredited this ideology, but since Obama took power just as it was happening, the old ways still had significant purchase, and new ones took time to take shape.

Therefore, a loud endorsement of deficit spending against austerity — with special attention for Clinton's $275 billion infrastructure plan — would go some distance to shoring up her left-wing youth support. All she needs to do is ditch the party's last vestiges of deficit scoldery, and embrace boldly Keynesian policy. Libertarian Gary Johnson is particularly vulnerable to attack on this front, because libertarian economics is basically the Ryan Budget on steroids. I would guess that he's picked up some youth support due to his aw-shucks demeanor and endorsement of legalizing marijuana, but that probably won't stand a close investigation of his brutal balanced budget plan.

Clinton ought to be able to win enough young voters to take the White House. She just needs to stop splitting her own base with overtures to conservatives.