Joe Biden and Donald Trump will both travel to the US-Mexico border on Thursday, locked in a high-stakes political duel on an issue that could ultimately decide the US presidential election.

That border was crossed last year by 2.5 million undocumented migrants, an influx that has overwhelmed processing facilities and pushed social services in major American cities to the brink.

The day provides an opportunity for Mr Biden to try to convince voters he is serious about immigration, while Mr Trump's own trip is yet another chance to shine a spotlight on an issue that has been the central focus of his political career.

November's general election is expected to be a Biden-Trump rematch, although the two candidates have not secured their respective parties' nomination quite yet.

Mr Trump is visiting Eagle Pass, the Texas border town where Republican Governor Greg Abbott has defied the Biden White House by using state National Guard soldiers to detain undocumented migrants and erect border barricades, including razor-wire fences that critics say are inhumane.

The former president is likely to tout these kind of aggressive measures and cite them as part of the reason why border crossings have dropped in Texas recently, while spiking in Arizona and California - states with Democratic governors.

The White House only announced Mr Biden's own visit to Brownsville, Texas, a few days ago and the president's trip is another indication that Democrats are scrambling to respond to an area of perceived weakness.

More than 6.3 million migrants have been detained crossing into the US illegally during Mr Biden's time in office - a higher number than under previous presidencies - though experts say the reasons for the spike are complex, with some factors pre-dating his government.

"He needs to get down there, show his face, and get the pulse of what's happening," says Jaime Dominguez, a professor of politics at Northwestern University. Mr Biden has been criticised for failing to engage on this issue until now, he notes, and "perception is reality".

That perception is translating into public opinion polls that paint a dark picture for the president. According to a recent Gallup survey, 28% of Americans named immigration as their top concern, beating out every other topic, including the economy and inflation. A Harris poll found Mr Biden's approval rating on the issue at 35% - his lowest issue rating.

Some 61% of Americans in a Monmouth survey listed illegal immigration as a "very serious problem", with a majority of respondents for the first time saying they support Mr Trump's proposal of building a US-Mexico border wall.

Leaders in major Democrat-run US cities have grown increasingly critical of the president's immigration policies - a consequence of the hundreds of thousands of migrants who have arrived in places like Chicago, Los Angeles and New York either on their own or with transportation arranged by Republican governors in states like Texas.

"Very progressive mayors are having to grapple with this issue, and they're pleading with the federal government to do something," Prof Dominguez says. "This isn't an issue Democrats can just hide behind and say that it's OK."

Mr Biden's border visit, the second of his presidency, appears part of a concerted effort to reverse this trend and turn the tables on Republicans - or at least minimise the political damage - allowing the election outcome to hinge on other topics, such as the economy or abortion rights.

The Biden camp has been hitting Mr Trump and congressional Republicans for blocking Senate-passed bipartisan immigration reform legislation in the House of Representatives earlier this month. They cite claims by the former president that he wanted to deny Mr Biden a victory on border security as evidence that Republicans are not serious about addressing the issue.

"Democrats called the bluff of the Republicans who for 30 years have said we need border security," says Douglas Rivlin, senior communications director for the pro-immigration group America's Voice. "They walked away because they're not interested in actually resolving these issues, they're interested in demonising immigrants because they see that as an important political strategy."

Mr Rivlin notes that Republicans tried to capitalise on immigration fears in recent national elections - including 2018, 2020 and 2022 - with minimal success.

Another prong of the White House's pivot on immigration may be tougher border measures and more stringent asylum policies that the administration has hinted the president could announce in the coming days. Such steps would be an effort to blunt the criticism that the administration has not done enough over the past three years to address what the president himself recently called a "crisis".

But this risks alienating pro-immigration elements of Mr Biden's political base, which could further fracture a electoral coalition that is already strained because of the president's support of Israel in its war in Gaza.

"If we're just talking about the border, and using that as a backdrop for the president's speech, and if he's a just adopting Trump talking points, it's not going to work for the president," Mr Rivlin says. "He has a potential to anger people in his own base without really persuading anybody that he's as tough as any Republican on the border."

Meanwhile, Mr Trump and the Republicans are pressing their perceived advantage. They criticise Mr Biden's efforts as too little, too late, they deride his border visit as a copycat move, and they say the bipartisan congressional reform package that Democrats supported would have been ineffective at best.

"Conscious, deliberate choices made by the Biden administration created what's going on down at the border right now, and the Biden administration is having to deal with the consequences of it," says Eric Ruark, director of research for NumbersUSA, a group advocating for lower immigration levels.

He says the Obama administration faced a similar surge in migrants and changed course. With Mr Biden, there was no one in the White House to "put the brakes on" until recently, as the general election campaign loomed.

"At some point, they realised that they have to at least give the impression that they're changing course," he says. "Whether they can sell that is the big question."

Mr Trump has his own immigration message to sell, and it is one that has its own set of weaknesses. When he was president, his early restrictions on immigration from majority-Muslim countries - an attempt at implementing his so-called "Muslim ban" campaign promise - created chaos at US airports and became mired in months of legal battles.

A 2018 policy of separating children and parents in families detained at the border was roundly denounced as cruel - and led Mr Trump to reverse course.

Now the former president is promising that if he is re-elected he will initiate an even more intense effort to combat undocumented migration, including enforcement efforts throughout the US and massive detention camps on the border.

Mr Rivlin calls that right-wing extremism which the Biden campaign should target for attack.

"Trump is talking about massive roundups and deportations," he says. "That doesn't really address where most Americans are. Most Americans want a secure border, but they also think that legal immigration is a good thing."

Thursday's Texas trips are just the beginning of what promises to be a pitched general election debate over immigration policy. There is still time for the political ground to shift, but given the state of public opinion, Mr Trump starts the fight with a clear advantage.

"There are steps to take that could do a lot to stem the flow right now," says Mr Ruark. "But the issue is we've got millions of people who are already in the country. It took three years to get here, and you're not going to solve it before the election."