Donald Trump has reversed course and endorsed House Speaker Paul Ryan for re-election.

The development comes as he seeks to stop his presidential campaign's unravelling after several self-goals and rising criticism from his own Republican Party.

"I need a Republican Senate and a House to accomplish all of the changes that we have to make," Trump said during a rally in Green Bay, in northern Wisconsin, Ryan's home state on Friday.

"We will have disagreements, but we will disagree as friends," Trump said.

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Trump made a plea for unity on Friday and pledged to work with the very Republican leaders he had earlier dismissed as Washington's ineffective establishment figures.

"We need unity. We have to win this election," Trump told the rally, as he stressed a "big tent" Republican Party is the only way to defeat Democrat Hillary Clinton in November's election.

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"In our shared mission to make America great again, I support and endorse our Speaker of the House, Paul Ryan," he said to cheers from the Green Bay crowd.

He also announced support for two senators, John McCain and Kelly Ayotte, two Republicans facing tough re-election fights.

Ryan, on the other hand, said Trump could cross a line that would prevent him from backing his own party's presidential candidate.

But "where that line is, I don't know", he said on Friday.

Trump has suffered a harsh campaign week that included his refusal to back down in a confrontation with the parents of a Muslim American soldier killed in Iraq.

Ryan, McCain and Ayotte had all criticised Trump for his remarks.

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On Friday, Michael Morell, a 33-year veteran of the CIA, who served presidents of both parties and headed the agency in 2011, announced his support for Clinton.

"Donald J Trump is not only unqualified for the job," he wrote in a New York Times column, "but he may well pose a threat to our national security."

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Morell said that as Russian President Vladimir Putin "played upon Mr Trump's vulnerabilities by complimenting him. He responded just as Mr Putin had calculated".

Trump has praised Putin as a great leader, and taken policy positions "consistent with Russian, not American, interests", including endorsing Russian espionage against US figures like Clinton, Morell said.

"In the intelligence business, we would say that Mr Putin had recruited Mr Trump as an unwitting agent of the Russian Federation," he said.

Morell also criticised Trump for his proposed temporary ban on Muslims entering the country.

"This position, which so clearly contradicts the foundational values of our nation, plays into the hands of the jihadist narrative that our fight against terrorism is a war between religions," he said.

He disclosed, without giving a name, that a Muslim American ran CIA's Counterterrorism Centre for nearly a decade after the September 11, 2001, attacks.

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Trump admitted Friday that he wrongly claimed to have seen secret Iranian footage of $400m in hard currency being delivered to Tehran as payment for the release of US prisoners.

He raised eyebrows this week when he made that assertion and gave details of what he said he saw in the video.

But he made a rare backtrack on Twitter.

Trump's campaign has insisted that the money was "a payment for hostages" held in Iran who were released in January.

The White House says it was returning cash from an unfulfilled 1970s Iranian military order.

Republicans are urging Trump to swiftly get back on message and train his fire on Clinton.

During a rally in Iowa, he branded her a "pathological" liar for her contortions about her emails and said the former secretary of state is "close to unhinged".

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If Clinton wins the White House, he said, "you will have really, in my opinion, the destruction of this country from within".

Meanwhile, Clinton is opening substantial leads over Trump in opinion polls.

A national McClatchy-Marist survey released on Thursday showed Clinton with a 15-point advantage, 48 percent to 33 percent.

In Georgia, which has voted Republican in every presidential election since 1996, Clinton leads by four points, according to an Atlanta Journal-Constitution poll.

Source: Al Jazeera And Agencies