President Donald Trump and Democratic White House hopeful Joe Biden are previewing a possible election duel over the world's most important geopolitical relationship.

It all starts with who knows Chinese President Xi the best. And as he consolidates his power by the month and masterminds China's rise to superpower status, it's a good bet that Xi is watching very closely.

Trump invoked the name of the most powerful Chinese leader since Mao Zedong on Friday as he probed for vulnerabilities in the armor of Biden who launched his campaign a day earlier.

"Sleepy Joe. He's a pretty sleepy guy. He's not going to be able to deal with President Xi," Trump told reporters.

Trump is sure to argue -- if he closes a mammoth trade deal with China -- that he has mastered relations with the Asian giant, a potentially useful 2020 reelection argument.

But his attack may backfire -- because the former vice president knows Xi as well, if not better, than Trump does.

He was tasked during the Obama administration with courting then-Vice President Xi during his apprenticeship for China's top job.

"I've spent as much time with Xi Jinping as anybody has," Biden boasted on the midterm election trail -- as he polished globe-trotting credentials that also saw him serve for years as the chairman of the Senate Foreign Relations Committee.

Biden and Xi traveled together in China. Then in 2012, they lunched together in Washington on soy-marinated Alaskan butterfish and gingered Swiss chard before hitting the road for Los Angeles.

Biden remembered his and Xi's travels through China and the US -- and bemoaned his decision not to travel with his guest to first-in-the-nation Iowa, on a return visit to the Hawkeye State for Xi, who was there as a young agricultural official in 1985.

"When he went to Muscatine, Iowa, I told him I couldn't go -- but I should have gone. He went, and he became President. I didn't go and I'm still Vice President," Biden quipped at a another State Department reception for Xi -- who was by then Chinese President -- in 2015.

Historic shift in US-China relations

JUST WATCHED Trump: US-China trade talks going 'very well' Replay More Videos ... MUST WATCH Trump: US-China trade talks going 'very well' 00:39

With their buddy boasts about Xi, Biden and Trump may be offering a taste of a testosterone heavy general election race should the veteran Democrat win the nomination.

But the banter underscores the new political significance of the increasingly acrimonious relationship between the world's most powerful nation and its emerging challenger.

Whoever eventually emerges as Trump's Democratic opponent will have no choice but to have a robust China policy to compete with the President's own aggressive approach.

The next election will take place after one of the most significant shifts in US-China relations since President Richard Nixon's historic trip to China in 1972.

For years, the goal of US policy was to manage China's entry into the global economy in the hope of heading off a hegemonic confrontation between Washington and Beijing.

Under Xi, China has been acting with increasing self-confidence in Asia, building up its military and is spreading its economic and diplomatic wings in Europe and Africa.

US efforts to frame a competing US-friendly regional trade and regulatory network were hampered by Trump's decision to exit the Trans-Pacific Partnership pact soon after taking office.

In its new National Security Strategy document, the Trump administration branded China, along with Russia as a "revisionist" power that uses "technology, propaganda and coercion to shape a world antithetical to our interests and values."

The 'King' of China

JUST WATCHED Trump warns China: 'I'm a tariff man' Replay More Videos ... MUST WATCH Trump warns China: 'I'm a tariff man' 01:47

Trump's approach to China has been mercurial.

On the campaign trail, he slammed China for "raping" US workers and blasted previous Presidents as soft touches who had let Chinese firms steal millions of US industrial jobs.

In office, Trump has escalated tensions by slapping $250 billion in tariffs on Chinese goods, drawing reprisals from Beijing. The two powers have been on the verge of a trade war for months. And he claim to have secured significant help from China into pressuring North Korea over its nuclear program.

Yet Trump has bonded with Xi personally, drawn to strongman qualities that have seen him melt in the company of autocrats such as Russia's Vladimir Putin and North Korea's Kim Jong Un.

Trump said earlier this year that he told Xi during a state visit to China in 2017 that his power was regal.

"He said, 'But I am not king, I am president.' I said 'No, you're president for life, and therefore you're king,' " Trump said. "He said, huh. He liked that. I get along with him great."

The test for Trump is whether that personal connection can deliver something tangible for the United States.

Trump is not the first President to complain about Chinese currency policy, the vast trade deficit and intellectual property theft and cyber espionage.

But if he can forge a breakthrough with China on those issues he could deliver a significant diplomatic achievement that eluded his predecessors.

Many in the US business community however expect a more limited agreement -- perhaps including large Chinese purchases of US goods to narrow the trade gap -- that Trump will hail as historic nonetheless.

A significant number of grass roots Democrats and lawmakers back Trump's populist approach to trade with China -- one of the ways in which Democratic Party politics have changed since Biden left office.

Biden might have no choice but to adopt a similar duality to Trump in relations with Xi -- on the one hand touting his close ties to Asia's most powerful man while at the same time adopting a strong set of policies designed to counter Chinese action on trade, human rights and territorial disputes with US allies in the South China Sea.

But neither Trump nor Biden doubt the stakes.

"The history of the next 50 years is going to be largely based on how well our two countries, the United States and China, navigate this relationship," Biden told Xi in 2015.

Trump told Xi in the Great Hall of the People in Beijing in 2017 that "a great responsibility has been placed on our shoulders."