Joe Biden keeps his wide national lead over Donald Trump as swing states tighten

The conventions are over and each party's ticket has been made official. The COVID-19 pandemic is still infecting tens of thousands of Americans every day, and anti-racism protests continue some of them becoming violent. But public opinion has largely held steady, meaning Joe Biden remains the favourite to defeat Donald Trump in two months.

But his path to the White House might be becoming a little narrower, as polls in a number of swing states tighten.

The CBC's Presidential Poll Tracker, an aggregation of all polls in the United States, still shows Biden leading in enough states to win him the election.

His margin over Trump among decided voters has decreased by about one percentage point since before the Democratic and Republican conventions, but it still stands at 7.9 points nationwide a little more than double what the gap was between Hillary Clinton and Trump at this point in the 2016 campaign.

Despite two weeks of the parties' infomercials, the impact on voters appears minimal. While polls taken before and after the two conventions suggest some movement toward Trump, it is far from unanimous, as a number of polls also show Biden's lead has widened slightly since mid-August.

That's not good news for Trump. Conventions usually cause a noticeable swing in the polls. Trump cut his deficit with Clinton in half after the 2016 Republican convention, pulling almost even with the Democratic nominee before her party's convention re-established her advantage.

But the map has gotten a little redder over the last few weeks. On Aug. 18, when the Democratic convention was just getting started, the Poll Tracker awarded Biden 308 electoral college votes in states deemed "safe" or "likely" wins for the Democrats, meaning he led in them by at least five points.

Now, the Poll Tracker awards Biden 290 electoral college votes in these states. That's still more than the 270 needed to win the presidency, but his margin for error has gotten smaller.

Swing states looking swingier

While movement at the national level has been modest, it has been more significant at the state level and largely toward Trump.

Since Aug. 18, Biden, vice-president under Barack Obama,has increased his estimated margin over Trump in only six states. Four of them are states that are already safe for the Democrats and one of them, Montana, is solidly Republican.

Only in Arizona has Biden improved his position in a contested state. His margin over Trump has increased by nearly two points, pushing his lead there to six points.

In the rest of the country, however, the Republican incumbent has made gains. And some of these states will be key to deciding the outcome of the election.

Ohio and Georgia, two states Trump won by comfortable margins in 2016, have turned three and two points redder, respectively. These are not important swing states for the Democrats, but they are absolutely essential for Trump if he is to have any hope of re-election.

Florida and Michigan, on the other hand, are big pieces of the Democrats' path. Both have also swung toward Trump by two points, with Biden now leading in Florida by just four points and in Michigan by six. If those trends continue, both states could move into toss-up territory.

Other states Trump won in 2016 but in which Biden is currently leading, like North Carolina, Wisconsin and Pennsylvania, have also gotten tighter over the last two weeks.

State to watch: Arizona

That Arizona has bucked this trend gives the Democrats some options on the electoral map. The state hasn't voted for the Democratic presidential nominee since 1996 and that has been the only time the party won Arizona since 1952.

But the state is becoming more diverse and, along with Texas, accordingly more Democratic. Barack Obama lost Arizona by eight percentage points in 2008 and nine points in 2012. Clinton, however, lost it by only 3.5 points.

This trend has continued for Biden. In 20 polls conducted in July and August, he has led in 16 of them.

Arizona is worth only11 electoral college votes, but that makes it as valuable as Minnesota or Wisconsin and nearly as much as Michigan. If Biden can win Arizona, that relieves some of the pressure on him to sweep the Midwest a region that has become more Republican-leaning than the country as a whole.

Texas, a big prize with 38 electoral college votes, has long been the Democrats' hope for a future swing state. It might not be a "purple"state just yet. But if demographically similar Arizona can turn blue, Biden can afford to let another state stay red elsewhere in the country.

In an election that could prove unpredictable and in which there are significant concerns about whether every vote will count Biden will need as many options as he can get.