President Joe Biden's pitiful performance on 27 June effectively knocked the incumbent out of the race for the White House.

Then on 10 September Biden's replacement, vice president Kamala Harris, proved she is a real contender, baiting her opponent Donald Trump into wild statements such as "they're eating the pets!".

The Democrats have recovered in the polls since Harris took over the nomination, including in so-called swing states, to the point that she is now narrow favourite to beat Trump, according to some respected analysts.

Others still reckon the Republican Trump will be re-elected. Either way, all agree the contest is on a knife edge with voting already under way in a handful of less populated states, and opening next week in Illinois.

With things so close, the televised debate next Tuesday could even tip the balance.

"All the needle needs to be moved is 0.1% in either direction, and that could be the difference in four or five states," according to Steven Maviglio, a Democratic strategist.

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Harris and Trump will not be on stage. This latest debate in CBS studios in New York City on 1 October is between their running mates, JD Vance and Tim Walz.

In most years vice presidential debates are sideshows which have little impact on the voters. Not this year.

The rise of Harris to presidential candidate has shown Americans that VPs are important. Just as Trump had to scramble to find a new running mate following stinging condemnation from Mike Pence, the man who served as his vice president for four years.

Adding to the excitement, the two men who will be facing off this week are also the best phrase-makers in this campaign.

Vance wrote bestselling book Hillbilly Elegy, based on his rough upbringing in the Appalachians. He likes to launch sweeping attacks on his foes, including dismissing Democratic women as "childless cat ladies". Taylor Swift embraced this jibe for herself in her recent post endorsing Harris.

Walz probably owes his place on the ticket to the single word "weird", which he spent the summer sticking on Trump and Vance to devastating effect in multiple media interviews on behalf of the Democratic campaign.

The confrontation between the two men promises to be spicy.

There is a generation gap between them. Walz is 60. Vance is 20 years younger. Walz likes to present himself as a folksy centrist dad. In The Manual, a signature campaign commercial, Walz sets about fixing his old car, "a '79 International Harvester Scout", while likening it to creating an opportunity economy for all.

He is also a veteran democratic politician having served 12 years in Washington in the US House of Representatives before being elected Governor of Minnesota in 2018, the post he still holds.

Vance's career has been meteoric. Four years in US Marine Corps provided his ladder to university. Then he became a corporate lawyer for investment firms.

Following the success of his book, his backers included the controversial tech titans Peter Thiel, Eric Schmidt and Marc Andreessen. After a lightning campaign in 2022, he is currently a first-term Republican US Senator for Ohio.

Read more:
Who is JD Vance?
Analysis: Why Harris picked Walz

Both men served in the military in non-combat roles. Vance was a journalist in uniform during his four years which included deployment to Iraq. Walz belonged to the Minnesota National Guard for 24 years.

The Harris campaign admitted he "misspoke" when he described assault rifles as "weapons of war that I carried in war".

The two "VP picks" share archetypal middle-American backgrounds, Nebraska and Minnesota for Walz and Kentucky and Ohio for Vance, which were major factors in why they were chosen as running mates. Harris is from California, Trump from New York City and Florida, all of which are regarded as coastal fleshpots by citizens in "flyover states".

Michigan, Wisconsin, Pennsylvania, Georgia, North Carolina, Arizona; the main candidates are all concentrating their campaigning on the battleground states - those most likely to "flip" decisively for one party or another, delivering a majority in the electoral college.

This weekend Walz has set up his debate camp in Michigan. In between mock debates in which the Transport Secretary Pete Buttigieg is standing in for Vance, Walz will meet and greet the locals in the bayside resort of Harbor Springs. Conveniently there is a "Festival of the Book" taking place which will allow Walz to strut his stuff as a school teacher.

Vance has called up US representative Tom Emmer from Walz's home state for his prep. The House majority whip should know where his old foe's vulnerabilities lie.

This debate will not be relaxed. Unusually for a vice presidential encounter, the protagonists will not be sitting down, they will be standing at lecterns. The last time that happened was 2008 with Sarah Palin and Joe Biden.

As with the other debates this year, the Presidential Debates Commission has not been called upon to organise this one. The two sides agreed their own rules with the broadcaster. This time there will be no studio audience, once again, and two moderators: CBS presenters news anchor Norah O'Donnell and Margaret Brennan of Meet The Press.

As Harris continues to challenge Trump to another debate without success, Vance has countered in advance demanding a second debate with Walz on 18 October. The Democrat is acting modest, protesting of Vance "he's a Yale Law guy. I'm public school teacher".

Walz hopes to play the part of a schoolmaster chiding a tearaway pupil. He will do well if he can emulate Lloyd Bentsen's crushing put down of the younger Dan Quayle in their 1988 vice presidential debate: "I knew Jack Kennedy. You're no Jack Kennedy."

Walz has fertile territory to exploit. JD Vance has already had to eat many of his wilder statements. He once likened his boss Trump to "Hitler". For electoral reasons he has U-turned on his book's thesis that his fellow poor whites were to blame for their own fecklessness.

Republican strategists hope that Vance will counter Walz's rebukes over sexism and abortion by sticking to mainstream issues such as inflation and immigration.

Vance can boast a nuanced personal record on some social issues including healthcare. But he is also pugnacious and may be unable to resist going after Walz aggressively for what Republicans regard as his left-wing voting record.

Trump's groundless claims that Harris is "a communist" seem to be impressing Hispanic voters.

Walz has more to lose and Vance has more to prove in the debate. Harris has embraced her choice of Walz, notably by appearing with him for her first major TV interview. Trump barely mentions Vance at his rallies. In opinion polls Walz has net approval ratings of 10%, Vance is at around minus 35%.

Debates are proving their value in this election year. Americans are paying increasing attention to them. 51.3 million tuned in to Biden/Trump earlier in the summer, 67.1 million watched Harris/Trump earlier this month.

An outcome on Tuesday night as vivid as in the two previous debates this year could well be a defining moment for the next presidency.

On the other hand, both veteran Democrats and Republicans will also remember that while Lloyd Bentsen smashed the debate, George H W Bush and Dan Quayle won the election.