Former president Barack Obama has re-entered the political fray nearly two years after leaving office, begging Americans to exercise their right to vote in the crucial November mid-term elections.

Speaking at a rally in Illinois, Mr Obama urged Democrats to vote to restore “honesty and decency and lawfulness” to government as the Democrats fight to regain key seats in the Senate and the House of Representatives.

“The biggest threat to our democracy is indifference,” he said on Friday afternoon [local time]. “The biggest threat to our democracy is cynicism. The cynicism led too many people to turn away from politics and stay home on election day.”

The former president, who has largely remained out of political life since leaving office, launched a blistering rebuke of Donald Trump during the speech at the University of Illinois and accused him of hijacking the Republican Party.

“It’s not conservative. It sure isn’t normal. It’s radical.”

Making his return debut on the campaign trail, the 57-year-old said this was the most important election he could recall in his lifetime.

The writer of the New York Times op-ed said Trump aides “are trying to do what’s right even when Donald Trump won’t”.

“Just a glance at recent headlines should tell you that this moment really is different. The stakes really are higher. The consequences of any of us sitting on the sidelines are more dire.”

Democrats need to win two seats in the Senate and 23 seats in the House of Representatives to regain control of both houses, which the Republicans currently control.

It’s understood Mr Obama will make appearances in several key states including California, Pennsylvania and Ohio, his legacy still looming large in a party divided over its future and with no clear presidential candidates ahead of the 2020 election.

The appearance came at the end of a disastrous week for President Trump, with revelations about his senior White House staff working against him detailed in journalist Bob Woodward’s upcoming book, Fear, and in an anonymous op-ed published by The New York Times.

Speaking to reporters on Air Force One on Friday, Mr Trump said he wanted Attorney General Jeff Sessions to investigate the source of the op-ed which has led to a witch hunt in Washington.

“I would say Jeff should be investigating who the author of that piece was because I really believe it’s national security.”

The president called the article, said by the Times to have been written by a senior White House official, as an act of treason.

“We’re going to take a look at what he had, what he gave, what he’s talking about, also where he is right now,” Mr Trump said.

Obama urges voting in high-stakes election, calling it a 'pivotal moment' https://t.co/bc2Z4DkdfA via @ReutersTV pic.twitter.com/ajUKQQijV4

— Reuters (@Reuters) September 7, 2018

Speaking at his own rally in North Dakota, Mr Trump accused his predecessor of trying to take credit for low unemployment levels and economic grow and derided Mr Obama’s speech as boring.

“I’m sorry, I watched it, but I fell asleep,” he said.

Concluding his hour-long speech, Mr Obama told the audience of mostly students that their generation was crucial in the fight to strengthen democracy in the United States.

“You can be the generation that at a critical moment stood up and reminded us just how precious this experiment in democracy really is, just how powerful it can be when we fight for it, when we believe in it.”

“I believe in you,” he said, promising he would “be right there with you every step of the way.”