MEXICO CITY (AFP) - The Central American migrants trekking toward the United States in a caravan have unwittingly become key players in Tuesday's (Nov 6) US midterms, though many are not even aware the elections are happening.

As Americans headed to the polls for a clutch vote seen as a referendum on Donald Trump's presidency, it was just another day on the road for some 4,500 migrants sleeping in tents, on bleachers and on the ground at a stadium-turned-shelter in Mexico City.

Having spent the night in the high-altitude chill of the Mexican capital, they made small talk, nursed nasty colds and feet mangled by weeks of walking, and sought to warm themselves as they stood in long lines waiting for breakfast.

Seeking to mobilise his base with hardline anti-immigration rhetoric, Trump has called the caravan a "national emergency," warned it was infiltrated by violent criminals and deployed some 5,000 active-duty troops to secure the US-Mexican border.

But as they rested and regrouped in Mexico City - still more than 1,000km from the border - most in the caravan were only vaguely aware of the outsize political dimension their trek had taken on.

"We don't have access to a lot of information on the road. I didn't even know" the United States was holding elections Tuesday, said Jairo Velazquez, a 24-year-old Honduran migrant.

Many in the caravan were more concerned with finding a phone or Internet cafe to talk with their families back in Honduras, Guatemala and El Salvador - the violent and impoverished "Northern Triangle" of Central America that they are fleeing.

Some just wanted to find something more to eat than the small portion of eggs and beans served by the Mexican authorities running the improvised shelter where they have paused to rest.

Others, however, had messages for the American president on voting day.

"Donald Trump is not the master of the Earth. The only master of the Earth is God," said Uziel Cantillano, 31.

Trump "needs to soften his heart and open the border, because all these people just want to work," he told AFP.

SAFETY IN NUMBERS

The caravan set out on Oct 13 from San Pedro Sula, Honduras, and was joined by other groups from Guatemala and El Salvador along the way.

Mexican authorities estimate it currently has about 5,000 people, some 500 of whom are still making their way across central and eastern Mexico to rejoin the main group.

Two other caravans with around 2,000 migrants each are currently making their way across southern Mexico. Another 3,000 Central Americans who were travelling by caravan have filed asylum requests in Mexico.

Hitch-hiking and walking, often in plastic shoes or flip-flops, sometimes pushing their babies in strollers, the migrants in the main caravan have covered more than 1,600km so far.

Many of them are fleeing poverty and insecurity in their home countries, where powerful street gangs rule their turf with brutal violence.

Immigration experts say it is highly unlikely they are travelling together with the goal of storming the US border - they are more likely to file individual asylum claims or try to sneak across in small groups.

Rather, they are seeking safety in numbers on a journey filled with risks. Mexican gangs regularly extort, kidnap or kill Central Americans trying to reach the United States.

'HARD-WORKING PEOPLE'

But images of this sea of Central Americans heading toward the United States and forcing their way across the Mexico-Guatemala border on Oct 19 have fed into Trump's politics, fuelling his message that his Democratic adversaries "want open-borders socialism."

Trump has alleged the caravan includes members of the violent street gang MS-13.

Mexican authorities said on Monday they had no evidence of criminals or people who could pose a security threat travelling in the caravan.

Some migrants, however, said they were aware of a small band of young gang members in the group.

But about 75 per cent of the caravan are "women, children, the elderly and other vulnerable people," and the rest "are mostly young men with their families," according to Gustavo Rodriguez Zarate, who helped host them over the weekend as head of migrant support programs for the Catholic archdiocese of Puebla.

"We're not criminals. We're hard-working people," said 25-year-old Honduran Eber Josue.

As they rested in Mexico City and planned their next move, some migrants were hopeful the US elections could change their plight.

"If they elect a new Congress, maybe that will give us an opportunity to reach the United States," said Carlos Rivera, 25, a Honduran man wearing a cap with an American flag.