In a surprise rebuke, Ryan announced on Thursday that he was "just not ready" to support Trump, who became the presumptive Republican presidential nominee this week after a big win in Indiana forced his remaining rivals from the race. Ryan told CNN that his party should have a leader who "appeals to a vast majority of Americans".

Just before Ryan's stunning statement, Trump attempted to make that kind of appeal - in his own eyebrow-raising way. The billionaire businessman sent a tweet from his Trump Tower office featuring a picture of him eating a taco bowl and declaring his affection for one of America's fastest-growing and politically powerful minority groups.

"I love Hispanics!" Trump wrote. He noted that he sent the message on Cinco de Mayo, a holiday marking the Mexican Army's victory over the French in 1862.

Trump's ode to the taco bowl may have been sent with good intentions. But it reflected a political tone deafness that worries some Republican leaders as they not only weigh the real estate mogul's prospects in the general election, but also how far they should go in linking themselves to his candidacy.

To some Hispanic leaders, Trump's tweet was offensive and patronising, the latest stumble from a candidate who has deeply offended many in their community.

"Eating a taco or wearing a sombrero doesn't cut it in our community in 2016," Janet Murguia, president of the National Council of La Raza, the nation's largest Hispanic civil rights organisation, said on Twitter.

Vicente Fox, the former president of Mexico, who has clashed with Trump over his signature proposal to build a wall between the two nations, joked that he saw that Trump is "now having some enchiladas and some Mexican food. I hope that he will not get indigestion".

To be sure, Trump's enthusiastic support from Republican primary voters - they skew whiter and older - is rooted in part in his willingness to cross lines of political correctness and flout traditional campaign decorum. But the voters Trump will face in his November race, likely against Democratic front-runner Hillary Clinton, will be far more diverse.

Trump already enters the general election struggling mightily with Hispanics, many of whom strongly oppose his call for building a wall along the US-Mexico border and deporting the millions of people in the United States illegally.

In an Associated Press-GfK poll conducted in early April, 82% of Hispanics had an unfavourable opinion of Trump and just 11% had a favourable opinion. At least 7 in 10 Hispanics said words including honest, civil, compassionate, competent and likeable don't describe Trump even slightly well.

Immigration laws

Ryan, a Wisconsin lawmaker and the Republican Party's vice presidential nominee in 2012, has been among the GOP leaders pushing for an overhaul of US immigration laws. He has also supported measures that would give legal status to those already in the country illegally.

Some Republicans see immigration as a threshold issue for Hispanic voters, and say their party stands little chance of having broad appeal with Hispanics unless it addresses the nation's fractured approach to immigration. Mitt Romney won less than 30% of Hispanics in his loss to President Barack Obama in the 2012 election.

Those stark political realities pose one of the biggest problems for Ryan and other GOP officials trying to gauge how supporting Trump in the short-term could impact their party - and, of course, their own political futures - in years to come. That's a particular concern for Ryan, who passed on running for president this year, but is already being eyed for a run in 2020.

Other prominent Republicans took similar steps to distance themselves from Trump this week, including former Presidents George HW Bush and George W Bush, who both made clear they would not be helping their party's nominee. But as the nation's highest elected Republican, Ryan's words have particular resonance and could give others in his party cover to take a similar stance.

Of course, Ryan may yet come around on Trump; he told CNN "there's a lot of questions conservatives are going to want answers to".

One of the first big-name Republicans to back Trump, New Jersey Governor. Chris Christie, said on Thursday: "Donald's got work to do to bring people together. I'm going to reach out to the speaker and see what his concerns are."

But Trump made clear later on Thursday that he won't be waiting around.

"I am not ready to support Speaker Ryan's agenda," Trump said. "Perhaps in the future we can work together and come to an agreement about what is best for the American people."