Double act or double trouble? Harris uses Biden sparingly on trail

Joe Biden may be out of the White House race but US voters haven't seen the last of him, as Kamala Harris makes careful use of her boss on the campaign trail.

It's less than six weeks since the 59-year-old vice president replaced the 81-year-old president as the Democratic contender, breathing new life into the party's election hopes.

But instead of fading out, Biden is taking on a targeted role in Harris's campaign against Donald Trump, focused on certain battleground states and demographics.

They've even appeared together, with an energetic Biden and Harris competing to flatter each other at an event in the Rust Belt city of Pittsburgh to mark the US Labor Day holiday.

"If you elect Kamala Harris as president, it will be the best decision you will have ever made," Biden told a cheering crowd as he introduced her, before she praised him as "one of the most of transformative presidents" in US history.

Afterwards, they posed for selfies with supporters before Biden invited Harris into his limousine to the airport, where their planes -- Air Force One and Two -- were parked next to each other.

"I would guess that the calculation in the Harris campaign is that he (Biden) can do some good in midwestern Rust Belt states," John Mark Hansen, a professor of political science at the University of Chicago, told AFP.

Indeed, Biden is heading on Thursday to two of those states -- Wisconsin and Michigan -- where he remains popular with older, white, blue-collar voters.

- 'Wiggle room' -

But the question is -- how much Biden is too much?

The vice president faces a dilemma as she seeks to keep up the wave of excitement since she replaced Biden at the top of the ticket.

"I think that there's a fine line to walk for her," Casey Burgat of George Washington University told AFP.

Stick too close, and voters may just see her as a sequel to the 81-year-old president, instead of the breath of fresh air her campaign has made her out to be.

But to cut herself off from Biden would be seen as disowning the same policies that Harris endorsed for four yeaars as his understudy.

"She needs him to be able to co-sign that she's ready for the job and that she's strong. But she also needs a little bit of wiggle room between her and the Biden-Harris administration," said Rebecca Gill, a political science professor at the University of Nevada, Las Vegas.

The historic circumstances of Biden's late withdrawal have also created a unique dynamic between president and vice president.

The presidential shadow is often heavy, with just six vice presidents having been elected to the highest office.

The last incumbent "veep" to do so was George H.W. Bush in 1988.

- Biden surge -

Harris, however, is aiming to succeed a one-term president.

The last similar example was in 1968, when Lyndon Johnson stepped aside as candidate amid the Vietnam War -- albeit only to see his vice president Hubert Humphrey lose to Richard Nixon.

With Harris, Biden has vowed to do "be on the sidelines" but to do "everything I can to help."

Gill said the sidelines might be the best place, given that Harris is still something of an unknown quantity to most Americans.

"Having Biden front and center might contradict some of that 'getting to know you' phase that a lot of voters are in with Kamala Harris," she said.

But there's one surprise development that may dispel concerns about a double act -- the historically unpopular Biden is seeing some of his best polls in years.

A USA Today/Suffolk University poll on Tuesday showed his approval rating rising to 48 percent from 41 percent just before his candidacy-ending debate with Trump in June, and a number of other polls have shown similar bumps.

"He is more popular now than he was when he was running," said Gill.