Former Texas Congressman Beto O'Rourke speaks to a crowd of marchers during the anti-Trump 'March for Truth' in El Paso, Texas, on February 11, 2019.

Democratic presidential candidate Beto O'Rourke will hold a major rally in El Paso, Texas, on Saturday, a city thrust to the center of America's immigration debate by President Donald Trump and the U.S government this week.

O'Rourke, a former congressman from El Paso, will kick off three rallies in Texas in his bid to become the Democratic nominee a day after Republican Trump threatened to close the U.S border with Mexico as soon as next week.

His rally in El Paso, which sits on the border with Mexico, has been long planned but the city became central to America's immigration debate this week.

U.S. Customs and Border Protection Commissioner Kevin K. McAleenan told a news conference in El Paso on Wednesday the southern border system was at breaking point because of the number of illegal immigrants crossing the border each day.

Trump, who says he is still determined to build a barrier along parts of the southern border, said on Friday: "There's a very good likelihood that I'll be closing the border next week, and that will be just fine with me."

He has repeatedly said he would close the U.S. border with Mexico during his two years in office but has not done so.

Trump and O'Rourke held dueling rallies in February in El Paso, which is already divided from Mexico by steel fencing.

Trump wants it reinforced and hundreds of miles of additional fencing built along the border. O'Rourke opposes any new border structures and opposition to Trump's border wall and immigration policies has been a centerpiece of his campaign.

They will again be a major part of his speeches in Texas on Saturday.

O'Rourke, who announced his White House campaign on March 14, shot to national prominence last year in an unexpectedly close race against incumbent Texas Republican Senator Ted Cruz.

His Texas rallies will be watched via livestream at more than 1,000 locations across America, according to his campaign.

O'Rourke, 46, competes in a large Democratic field. More than a dozen candidates have joined the fight to become the candidate to take on Trump in 2020.

The O'Rourke campaign sent multiple requests to potential supporters for campaign donations before his rallies in El Paso, Houston and Austin on Saturday, a common practice among presidential hopefuls. The messages stressed the importance of donating before Sunday, the deadline for first quarter fundraising reports with the Federal Election Commission.

O'Rourke smashed fundraising records as a Senate candidate and raised $6.1 million in the first 24 hours of his presidential campaign, the largest first-day haul of any announced candidate this year.

However, he has struggled to see a strong campaign work ethic translate into a significant boost in early polling.

O'Rourke trails former vice president Joe Biden and Vermont Senator Bernie Sanders by double digits, according to early polls among Democratic voters. Analysts warn that polls this early, before the first nominating votes are cast in Iowa in February 2020, are unreliable.

Biden has yet to join the race, although he is expected to announce his presidential candidacy soon.