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Conversations in which Democrat Hillary Clinton is advanced as the odds-on favourite to win the US presidency usually contain an explicit or implicit "but".

Yes, Mrs Clinton has built-in advantages in electoral maths and organisation and Donald Trump's controversial views and propensity for wandering woefully off-message are political liabilities - but what if there were a headline-grabbing Islamic militant attack on US soil before November's election? Would that significantly alter the state of the presidential race?

We may be about to find out.

Mr Trump was quick to take to Twitter after news spread that the assailant in the mass shooting at a gay nightclub in Orlando, Florida, was a child of Afghan immigrants and had apparently "pledged allegiance" to so-called Islamic State.

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Like many Trump tweets, it instantly provoked both praise from his supporters and outrage from others including gay rights activist George Takei.

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Following Barack Obama's short speech in which the president called the Orlando attack an "act of terror and an act of hate" and said it was "a further reminder of how easy it is for someone to get their hands on a weapon that lets them shoot people", Mr Trump doubled-down on the rhetoric that has fuelled his rise to the Republican nomination.

He called for Mr Obama's resignation for his "refusal to even say the words 'radical Islam'".

"If we do not get tough and smart real fast, we are not going to have a country anymore," he wrote. "Because our leaders are weak, I said this was going to happen - and it is only going to get worse. I am trying to save lives and prevent the next terrorist attack. We can't afford to be politically correct anymore."

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Left to the imagination is what, exactly, that means. Should the children of second-generation Muslim immigrants - like the alleged assailant, Omar Mateen - receive special screening? Trump critics will surely assume the worst. His allies will imagine the greatest.

Mr Trump went on to assert that Mrs Clinton - who supports resettling 65,000 Syrian refugees in the US - wants to "dramatically increase admissions from the Middle East" and that the US has "no way to screen them, pay for them, or prevent the second generation from radicalising".

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Meanwhile, Mrs Clinton sent out her own press release - which largely dovetailed with Mr Obama's public statement.

"We need to redouble our efforts to defend our country from threats at home and abroad," she wrote on Facebook. "That means defeating international terror groups, working with allies and partners to go after them wherever they are, countering their attempts to recruit people here and everywhere, and hardening our defences at home. It also means refusing to be intimidated and staying true to our values."

She also told the LGBT community that she was one of their allies and renewed her call for new gun control measures, saying that "weapons of war have no place on our streets".

Mrs Clinton has made gun control a key talking point for much of her campaign - the one area in which she was successfully able to position herself to the left of her Democratic Socialist opponent, Bernie Sanders. Her latest statement is part of an attempt to frame these views as a national security issue.

Media playback is unsupported on your device Media caption President Obama: "We are united in grief, outrage - and resolve"

Such a strategy has yet to work for Mr Obama and congressional Democrats, who advanced similar arguments after the attack last December in San Bernardino, California. Gun control in the US - at least up until now - is one of the areas where the partisan divide is most firmly entrenched.

The duelling press releases and social media posts from the two major party presumptive nominees puts immigration, militant attacks and gun control squarely in the centre of the general election.

Mr Trump had been quietly backing away from his call for a temporary ban on Muslim entry into the United States. Now he is explicitly referencing it - even boasting of his prescience.

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He spent most of his California victory speech last Tuesday focusing on his economic message and previewing attacks on the Clintons he said he would expand on in a speech this Monday.

Now that speech will be on "this terrorist attack, immigration and national security".

Mr Trump appears to be calculating that the American public will determine that, after Orlando, the status quo is untenable; that he has the upper-hand in a contest between his unprecedented border-security and immigration proposals and Mrs Clinton's calls for moderation, continuation of Mr Obama's foreign policy and increased regulation of firearms.

Just yesterday, the prospect of the election turning on such a face-off was purely theoretical. Now it's very real.