Bernie Sanders is returning to the campaign trail for Hillary Clinton, making several appearances at college towns through Ohio where she appears to be in a dead heat with Donald Trump.

With so much at stake in the Buckeye State, Mr Sanders has been given the key mission over this weekend of firing up the energy of younger voters, with whom he was so popular when he gave Ms Clinton a run for her money during the primaries before she clinched the nomination.

It is an effort that won’t be left to the Democratic Socialist Senator from Vermont alone. Also making stops in Ohio this weekend will be Senator Elizabeth Warren of Massachusetts.

Both figures, popular with the progressive left of the Democratic Party, have agreed to help defeat Mr Trump in what remains of the election campaign, even if they have both put Ms Clinton on notice that they expect her to give more than lip service to their liberal ideals.

“He is pathetic,” Mr Sanders said of Mr Trump on Friday, commenting on the Republican’s meandering on the issue of whether President Barack Obama was or was not born in America. “This goes to the root of what Trump’s campaign is about – it’s about bigotry,” he told CNN.

The deployment of Mr Sanders and Ms Warren to Ohio is a sign, however, of growing nervousness in the Clinton campaign of a recent narrowing of the national polls and polls in swing states. Keeping Mr Trump at bay in states like Florida and Ohio – no Republican has won the White House without winning Ohio first – is crucial for Ms Clinton’s campaign.

US election 2016: the race for the White House in pictures Show all 12 1 /12 US election 2016: the race for the White House in pictures US election 2016: the race for the White House in pictures Republican U.S. presidential nominee Donald Trump shakes hands with Democratic U.S. presidential nominee Hillary Clinton at the conclusion of their first presidential debate at Hofstra University in Hempstead, New York Reuters US election 2016: the race for the White House in pictures President Barack Obama embraces Democratic nominee Hillary Clinton on stage at the party's convention in Philadelphia US election 2016: the race for the White House in pictures Donald Trump's wife Melania delivered a speech at the GOP convention in Cleveland that was later found to have been cribbed in part from Michelle Obama's 2008 convention address AP US election 2016: the race for the White House in pictures Hillary Clinton talks to reporters aboard her new campaign plane on Labour Day, 5 September, her first 'press conference' since 2015 (Getty Images) US election 2016: the race for the White House in pictures Donald Trump held a joint press conference with Mexican leader Enrique Pena Nieto in Mexico City in August, hours before reiterating his harsh immigration plans at a campaign rally in Arizona Reuters US election 2016: the race for the White House in pictures Bernie Sanders officially endorsed Hillary Clinton, saying his progressive vision for ‘a transformed America’ would be ’best served by the defeat of Donald Trump’ Reuters US election 2016: the race for the White House in pictures Khizr and Gazala Khan appeared at the DNC to slam Trump for his stance on Muslim immigration, citing the case of their son Humayun Khan, who was killed in combat while serving as a Captain in the US Army in Iraq US election 2016: the race for the White House in pictures Libertarian candidate Gary Johnson is doing better in polls than any third party candidate since Ross Perot, 20 years ago Nicholas Kamm/AFP/Getty US election 2016: the race for the White House in pictures Green Party candidate Jill Stein (centre) marches with supporters in Colorado AP US election 2016: the race for the White House in pictures Hillary Clinton and Virginia Senator Tim Kaine at a rally in Kaine's home state in July, days before Ms Clinton tapped him to be her running mate Getty US election 2016: the race for the White House in pictures Trump on the campaign trail with his vice presidential pick, Indiana governor Mike Pence AP US election 2016: the race for the White House in pictures Former Ukip leader Nigel Farage appears at a Trump rally in Mississippi in August, where he told the crowd that he 'wouldn't vote for Hillary Clinton if you paid me'.

In Ohio, both senators will be emphasising Ms Clinton’s education plans, in particular to eliminate college tuition fees for less well-off Americans and to make attendance at community college free, as well as the possibility of studying without accumulating debt, for every young person.

Mr Sanders’ tour has been billed as a “Weekend of Action” and will see him attending events in Canton, Kent and Akron in northern Ohio. He will also stress other issues popular with younger votes including action to combat climate change, widening access to health care and Ms Clinton’s commitment to raise the minimum wage.

“What every American – Democrat, Republican, independent – has got to ask themselves, issue by issue, [is], ‘Who is the better candidate for them, for the middle class, for working families?’” Mr Sanders told USA Today.

“And if you go through the issues – raising the minimum wage, pay equity, family leave, making public colleges and universities tuition free, climate change – on all of those issues and many, many others, clearly Hillary Clinton is far and away the superior candidate. That’s what has to be dealt with and that’s the point that we’ll try to make.”

As well as reminding millennial voters in Ohio of the basics of Ms Clinton’s platform, Ms Sanders will also be urging them to register vote before a mid-October deadline. While the Clinton campaign has vastly superior on-the-ground, get-out-the-vote organising infrastructures in all the swing states, there is still concern that she is suffering from an excitement deficit.

Concerns about the likely turn-out will be especially key where victories for Ms Clinton may rest in part on ensuring that minorities go to the voting stations on 8 November. That means firing up the African American vote in Ohio, especially in those northern urban centres like Akron and Cleveland. In Florida the campaign is also very focused on getting Hispanic voters to the polls.

That will also mean encouraging voters to engage in early voting in those states that allow it. “The time I want Bernie – and I love him – is for early voting,” Donna Brazile, the interim chair of the Demoratic National Committee, said. “Bernie Sanders is going to help Democrats up and down the ticket.”

In 2012, some 35 per cent of ballots were cast early. This year, 37 states and the District of Columbia allow some form of early voting. Indeed early voting will begin first in several states – Idaho, Minnesota, South Dakota, Vermont, Wyoming – on 23 September. In other words, in one week.