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Donald Trump said Monday that any suggestion the Russians hacked and leaked Democratic Party emails on his behalf is a "joke."

The timing of the embarrassing leak on the eve of the Democratic National Convention put fresh attention on relations between Trump and Russian president Vladimir Putin, who have praised each other, and on close ties between key members of Trump's inner circle and Russian businesses and officials.

Hillary Clinton's campaign manager alleged over the weekend that the leak was designed to help Trump after his campaign operatives pushed changes to the Republican convention platform "to make it more pro-Russian" — reportedly removing language in support of sanctions for Russia and new weapons for Ukraine.

In a tweet, Trump scoffed at the notion, writing, "The new joke in town is that Russia leaked the disastrous DNC e-mails, which should never have been written (stupid), because Putin likes me."

The new joke in town is that Russia leaked the disastrous DNC e-mails, which should never have been written (stupid), because Putin likes me — Donald J. Trump (@realDonaldTrump) July 25, 2016

Trump has repeatedly praised Putin as a great leader even as Republicans and Democrats have labeled him a thug and threat to global security. Putin, in response, has called Trump the “absolute leader of the presidential race” and “a very bright and talented man.”

Trump has downplayed Putin's invasions of other countries and alleged assassinations of enemies. "At least he's a leader," he told MSNBC's "Morning Joe" in December.

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Trump's position that he would not automatically support NATO allies is also appealing to Putin, who has threatened some of them. Trump has also said the U.S. should not lead an international effort to help Ukraine fight Russian intervention — and he advocates letting Russia fight ISIS in Syria despite Russia's support for the Bashar al-Assad regime.

Both Trump and his son Donald Jr. have traveled to Russia for business many times, and wealthy Russians have invested in Trump properties and ventures.

The candidate's campaign chairman, Paul Manafort, is a longtime adviser to senior pro-Russian leaders in Ukraine, including ex-president Viktor Yanukovych, and has worked for a billionaire oligarch there.

Trump adviser Carter Page has advised Russian energy giant Gazprom and says the U.S. is too anti-Russia. A former adviser, Michael Caputo, had a contract with Gazprom's media arm in 2001 to improve Putin's image in the United States.

Other Trump advisers with business ties to Russia include Richard Burt and Howard Lorber, who has traveled with Trump to Russia.

Michael McFaul, who was U.S. ambassador to Russia while Clinton was secretary of state, said there's no question in his mind about who Putin would like to see in the White House.

"Both he himself and his media outlets, and his surrogates, and people in his party have made it very clear they prefer Trump over Clinton," he told MSNBC's Chris Hayes.

Some of that support is because of Trump's policy statements, but some is personal, McFaul said, noting that Putin has accused Clinton of fomenting opposition protests by criticizing Russian elections in 2011.

"Ever since, he has had a vendetta against Clinton," McFaul said.

Retired Adm. James Stavridis, former head of U.S. European Command and NATO Supreme Allied Commander Europe, who was reportedly vetted as a possible Clinton running mate, called the Trump-Putin dynamic "a total romance."

In Stavridis' view, Putin thinks Trump would an easier adversary than Clinton would.

“I think they see her as someone coming from a position of someone who has attempted to have good relations with Russia. They found that too difficult and … are afraid that she will be a strong and determined interlocutor who will demand adherence to international law," he told NBC News.

"When they look at someone like Mr. Trump who expresses admiration for Putin, is kind of a dealmaker in his own way and shows, shall we say, more flexibility in how he views other nations — then I think he looks a lot more attractive than Secretary Clinton.”

The DNC email leak, which led to the ouster of party chairwoman Debbie Wasserman Schultz, remains under investigation by the FBI.

"A compromise of this nature is something we take very seriously, and the FBI will continue to investigate and hold accountable those who pose a threat in cyberspace," the bureau said in a statement Monday.

But some experts have said that the dump of 20,000 emails by Wikileaks on Friday, appears to have been made possible by the Russian government.

“This has all the hallmarks of tradecraft," Tom Kellermann, the CEO of Strategic Cyber Ventures told Defense One. "The Cold War is alive and well."