WASHINGTON/TAPACHULA, Mexico — President Donald Trump on Monday vowed to begin curtailing tens of millions of dollars in aid to three Central American nations and called a caravan of migrants bound for the United States a national emergency as he sought to boost his party’s chances in Nov. 6 congressional elections.

“Guatemala, Honduras and El Salvador were not able to do the job of stopping people from leaving their country and coming illegally to the US We will now begin cutting off, or substantially reducing, the massive foreign aid routinely given to them,” Trump wrote in a series of Twitter posts.

Trump and his fellow Republicans have sought to elevate the caravan, which has made its way into Mexico bound for the US border, and immigration as campaign issues ahead of the midterm elections, in which his party is fighting to maintain control of the US Senate and House of Representatives.

“Remember the Midterms!” Trump wrote in Twitter posts decrying the caravan and attacking Democrats on immigration.

Trump, who has taken a hard line toward illegal immigration since taking office last year, also wrote that he had alerted the US military and Border Patrol “that this is a National Emergy,” though he did not say what actions he was planning.

In campaigning for Republican candidates last week, Trump highlighted the caravan issue. Trump travels to Texas, a key border state, later on Monday to campaign for Republican US Senator Ted Cruz, who is seeking re-election.

Since Trump became president last year, the United States has already moved to sharply decrease aid to Central America.

In 2016, the United States provided some $131.2 million in aid to Guatemala, $98.3 million to Honduras, and $67.9 million to El Salvador, according to official US data. By next year, those sums were projected to fall to $69.4 million for Guatemala, $65.8 million for Honduras, and $45.7 million in the case of El Salvador. Combined, the cuts amount to a reduction of almost 40 percent for the three nations.

Trump was not specific about how much more he wanted to cut the aid.

At least 5,000 migrants, mostly Honduran, massed late on Sunday in the Mexican city of Tapachula near the Guatemalan border, local police said. About another 1,000 migrants were walking toward them from Ciudad Hidalgo further south, according to a Reuters witness.

Seeking to escape violence and poverty in their home countries, the migrants — men, women and children — have defied threats by Trump that he will close the US-Mexico border if they advanced, as well as warnings from the Mexican government.

Most of the migrants are expected to stay in Tapachula through Monday, according to Mexican police at the scene. Police in riot gear shadowed the caravan’s arrival along a southern highway but did not impede the migrants’ journey.

“Sadly, it looks like Mexico’s Police and Military are unable to stop the Caravan heading to the Southern Border of the United States,” Trump wrote on Twitter.

Lieutenant Colonel Jamie Davis, a Pentagon spokesman, said that while National Guard troops are currently supporting the Department of Homeland Security on the border, the Pentagon had not been asked to provide additional support. There are currently 2,100 National Guard troops along the border in Texas, New Mexico, Arizona and California, according to the Pentagon.

Representatives for the White House and the US Border Patrol did not immediately reply to requests for comment.

Congress has not fully funded Trump’s proposed wall along the US-Mexican border, which he has argued is needed to combat illegal immigration.

In April, he raised the prospect of sending active-duty military forces to the U.S.-Mexico border to stop illegal immigrants, raising questions in Congress and among legal experts about troop deployments on US soil.

A 19th-century federal law restricts using the Army and other main branches of the military for civilian law enforcement on American soil, unless specifically authorized by Congress. But the military can provide support services to law enforcement and has done so on occasion since the 1980s.

Later in April, Trump and Defense Secretary James Mattis authorized up to 4,000 National Guard personnel to help the Department of Homeland Security secure the border in the four Southwestern border states.

Some specific statutes authorize the president to deploy troops within the United States for riot control or relief efforts after natural disasters.

In a tweet on Monday, Republican US Senator Marco Rubio wrote: “While unlawful migration to US from Central America is caused by real crisis, the migrant ‘caravan’ was manufactured by supporters of a radical agenda who are using poor & desperate people to try & embarrass & undermine the US in the region. But it’s going to backfire on them.” He did not provide further details on who the supporters were.

Mexico’s government has said the migrants must register with authorities so their claims to asylum in Mexico may be assessed. That way, the government hopes to break up the train of people. If they do not qualify for asylum, those in the caravan are liable to be sent back to their home countries. — Reuters