Gary Johnson is embarking on the two most important weeks of his campaign for the U.S. presidency.

The Libertarian Party candidate wants to get a spot in the presidential debates, and the final decision on who gets in will be made soon. Whether he secures an invitation will have a significant impact on the fate of his party.

It will also play a big role in the campaigns of both Hillary Clinton and Donald Trump.

The independent Commission on Presidential Debates will hold its first debate on Sept. 26 in Hempstead, N.Y. In order to get a spot in that debate, Johnson will need to surpass the polling threshold of 15 per cent.

The commission will judge whether Johnson has crossed that threshold in mid-September. That likely means the polls that will decide Johnson's fate are already in the field.

According to the CBC's Presidential Poll Tracker, support for third-party candidates among decided voters stands at just under 13 per cent — a very high mark at this stage of the campaign. While that includes support for the Greens' Jill Stein and other third-party candidates, the bulk of it belongs to Johnson.

But the Poll Tracker is based on all published polls. The commission will only focus on five: those of ABC News/Washington Post, CBS News/New York Times, CNN, Fox News, and NBC News/Wall Street Journal.

Johnson needs 15 per cent support to join Trump and Clinton in the presidential debates. (Clinton: Aaron Bernstein/Getty, Trump: John Moore/Reuters) Only Fox News has published numbers recently . In its latest poll, it pegged Johnson at nine per cent support. The other four pollsters were in the field at the end of July and in early August. They are expected to release new numbers soon.

The pollsters the commission will be using to make its decision represent a good group for Johnson. When they were all last in the field, a month ago, Johnson averaged 9.6 per cent in their polls. Other pollsters then in the field — ones the commission will ignore — had him at just 7.3 per cent.

But it doesn't seem likely that when the five pollsters do report next they will put Johnson close enough to that 15 per cent to get him into the debates. Those pollsters the commission won't be taking into account have not shown Johnson to be any stronger today than he was a month ago. He's averaged 7.4 per cent, according to their latest soundings.

That is why the next few weeks will be critical for Johnson. He needs to get his poll numbers up. A sympathetic super PAC recently spent $1 million on advertisements to help him — a rare ad buy for a third-party candidate.

The Hempstead debate is Johnson's best chance to make an impact. If he doesn't get into that first debate, he's extremely unlikely to see his support increase enough to get him into the two that will be held in October.

Johnson a known unknown

Voters are more likely to focus on the candidates they will see on the debate stage. The more the campaign becomes a competitive contest between Hillary Clinton and Donald Trump, the less likely voters are to cast a ballot for a candidate like Johnson who has no realistic chance of taking the White House.

Johnson remains the biggest wildcard in this election. If he gets into the debates, the platform may boost his support at the expense of one or both of the other main candidates. Though he's a former Republican governor of New Mexico, he's a social liberal — which might appeal to Democrats.

But if he doesn't get into the debates and his support drops, it's unclear whether he will bleed more votes to Trump or to Clinton. The polls show mixed results.

That makes him a big unknown. What is known, though, is that Johnson's support — and his potential appeal with a bigger platform — is large enough to swing the election. Clinton's lead is now down to just four points.

In the end, the most important impact of the debates might not come from what is said, but simply who is on the stage.