WHITE PLAINS, N.Y. -- For months, Democrats argued that voters would get "serious" about the campaign once it reached the fall and would reject Donald Trump's no-holds-barred approach.

They're still waiting.

With fewer than 50 days left, polling shows a tightening national race and -- most unnerving to Democrats -- a Trump rise in key battleground states. But as Trump's provocative appeal gains traction, Hillary Clinton is sticking with the traditional playbook: Lots of attack ads, a focus on getting out the vote and intense preparation for next week's first general election debate.

Her approach underscores what's emerged as a central question of the 2016 campaign: Can Clinton's play-it-safe political strategy win against a chaos candidate?

Even President Barack Obama, who long dismissed the idea of a future Trump administration, has started ringing alarm bells, warning Democratic supporters to expect a tight race that Clinton could possibly lose. Recent polls suggest the Republican may have an edge in Iowa and Ohio and is likely in a close race with Clinton in Florida and North Carolina.

"This guy is not qualified to be president," Obama told donors at a Manhattan fundraiser on Sunday. "This should not be a close election, but it will be."

Clinton's campaign, Democrats say, has little choice but to stick with its plan. The always-measured Clinton, they argue, can't out-improvise one of the most unpredictable politicians of the modern era.

"We're going through the roller-coaster rides of campaigns. All she can do is just keep plowing ahead," said Steve Schale, a Democratic strategist who ran Obama's Florida operation in 2008 and advised him four years later. "She's going to win it by grinding it out."

Hoping to calm some supporters' concerns, Clinton's campaign sent out a memo Monday, reminding them that the electoral map favours Democrats. The memo charted various paths to 270 electoral votes and urged backers to channel their worry into volunteering.

"Battleground states carry that name for a reason: They're going to be close, from now until Election Day," wrote campaign manager Robby Mook. "But we are going to win them because we've spent the past year building a superior ground game to communicate our message and turn our people out to vote. So instead of worrying, let's just get to work!"

But Trump, who lacks Clinton's organized effort on the ground but regularly fills massive arenas, is far from a standard opponent. In the primary, he knocked off more than a dozen rivals who took a basically standard approach to his unpredictable rhetoric.

"Everybody in the primary at one point or another tried to take Donald Trump down in the way you take someone down who says absurd things and none of them worked," said Rick Tyler, a former aide to primary rival Sen. Ted Cruz. "She's trying to do more of the same. And more of the same isn't working."

Clinton aides see next week's debate at Hofstra University as a key moment. The Monday night match-up will finally give voters a chance to compare the candidates side-by-side.

Clinton must communicate the "contrast and choice to voters that are tuning in for the first time," said spokesman Brian Fallon.

For his part, Trump has begun taking baby steps toward becoming a slightly more traditional candidate, reading off teleprompters, rolling out policy proposals and making overtures to minorities -- creating even more uncertainty among Democrats about how he'll act on the debate stage.

Though aides decline to detail debate preparations, Clinton has built a lot of downtime into her schedule for recent weeks. Then there was the pneumonia episode.

She is holding sessions with experienced Democratic debate experts, including Ron Klain, Karen Dunn and Robert Barnett, all of whom advised Obama. One closely held secret: the identity of the person playing Trump in the sessions.

"In an unpredictable race against an unpredictable candidate, by definition the only thing you can control is what you do," said Mo Elleithee, a former Clinton aide who's now head of the Georgetown University Institute of Politics and Public Service. "They're focusing on that."

While Clinton has been prepping, her team has stuck with its strategy: Define Trump in the summer with a barrage of negative ads.

Clinton' campaign and allies have spent more than $180 million on TV and radio advertising between mid-June and this week, according to Kantar Media's political ad tracker. Trump and his supporters spent about $40 million in the same time period.

It's a strategy that mirrors the one pursued by Obama during his re-election campaign, when his team barraged Mitt Romney through the summer with ads casting him as an out-of-touch plutocrat.

But Clinton, with deep unfavourability ratings of her own, is a far different candidate from Obama. Her team is making a renewed push to ensure turnout from groups who supported the president -- young voters, Latinos and African-Americans. But she acknowledges she has work to do, telling young voters in Philadelphia on Monday she understands they "may still have some questions" about her.

Looking to the debates, Clinton says she's ready for whatever Trump sends her way.

"I am going to do my very best to communicate as clearly and - and fearlessly as I can in the face of the insults and the attacks and the bullying and bigotry that we've seen coming from my opponent," she said on "The Steve Harvey Morning Show." "I understand it's a contact sport."

Associated Press writer Julie Bykowicz contributed to this report