This has been the Summer of Trump on the campaign trail. Donald Trump has flown high in the polls, with seemingly nothing emerging to slow his rise.

But as heading into September, here are three hurdles the reigning Republican front-runner might have to contend with that run counter to his success so far:

1. Raising money (or pouring more of his own in) to try and launch a professional campaign operation. Trump continues to downplay the need for his campaign to raise money. But if he's serious about being in the race for the long haul, he's going to either have to start asking for money for real or he's going to have to dump some serious cash of his own in.

It has become clear that Trump's campaign and its allies are not shy about bringing in more dough. Late last week, he appeared at a campaign event where the price of admission was $100 (Trump insists it wasn't a fundraiser), and his campaign launched an online fundraising drive in August. Plus, the Trump superPAC — Make America Great Again (of course) — held a high-dollar dinner a couple of weeks ago at the Jersey Shore.

If Trump and his supporters are raising money, then his fundraising will be scrutinized. How much he's raised and from whom won't be known until quarterly reports are released in October, but those reports will reflect what candidates have raised by the end of September.

The big questions will be — how and when does Trump start trying to turn his big crowds and support from people who tell pollsters they're for him into actual voters? And does he start running TV ads?

2. Pledging loyalty. Perhaps the most politically significant hurdle for Trump this month is the impending deadlines for various primary contests. He has to file to get on the ballot in South Carolina by Sept. 30. The catch? In order to get on the ballot, Trump will have to pledge his support for the eventual Republican nominee.

He memorably kept the door open to an eventual third-party run at the first GOP debate. In Nashville over the weekend, he said he'll make a decision about South Carolina "very soon." Trump said once he makes his decision, "I think a lot of people are going to be very happy." It's hard to interpret that in any way other than Trump leaning toward signing the pledge. It's also hard to imagine any candidates going forward in a serious way without appearing on the South Carolina ballot — even if they don't expect to win there. Trump, of course, does expect to win, particularly since he's leading in the polls there currently.

When asked whether he'd launch an independent bid, Trump usually notes that he feels that the Republican Party has treated him fairly. Two other states — Virginia and North Carolina — are also exploring loyalty oaths and will have to submit their ballot requirements to the Republican National Committee by Oct. 1. It's possible Trump could feel targeted by such moves, as he famously felt targeted by the first question in the Fox News debate, and decide he's not being treated fairly.

Even if Trump goes ahead with the loyalty pledge, that may not be the end of that. His campaign manager was recently dismissive of the notion, and he noted to the Washington Post that people could see pledges as "a typical Washington, D.C., idea, to pledge to do something only to change our mind later." If conditions change down the road, and Trump feels he's not being treated fairly by the GOP, he could argue that the pledges are no longer binding.

3. Staying in the news — in the ways he wants. OK, don't laugh. Before you say this won't be a problem for Trump, who has rocketed to the top of the Republican field with enormous media attention, it could turn out to be something to watch. He's undoubtedly good for ratings and for clicks, and that won't change heading into the fall. But there will be a LOT more news to compete with.

Campaign news, including the second GOP debate on Sept. 16, will still get a lot of attention. But there's also Pope Francis' visit to the United States, the U.N. General Assembly, and a state visit to Washington by Chinese Premier Xi Jinping. That's not to mention other events that people outside of the Beltway will also care about, like the expected launch of the new iPhone, the New England Patriots' return to the field sans Tom Brady (oh, and just football in general) and Congress' return.

In Congress, serious issues loom, like a possible government shutdown fight over funding for Planned Parenthood, a push for cybersecurity legislation following high-profile hacks in the private sector and government, and the debate over the Iran deal.

There's a Sept. 17 deadline on the Iran deal for congressional review, and that's where Trump will try to employ his skill for staying in the headlines. He'll appear with GOP presidential rival Sen. Ted Cruz, R-Texas, next Wednesday at a rally against the deal in front of the U.S. Capitol.

Still, how does Trump respond when the issues are not only serious but specific?

So what? It would be foolish to predict that if Trump tripped over any of these hurdles it would damage his campaign. Nothing has yet. Still, the end of summer traditionally means the end of the political "silly season."

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