By Jeff Mason and Roberta Rampton

WASHINGTON (Reuters) – President Donald Trump said on Thursday he was finalizing a plan that would require immigrants seeking asylum in the United States to approach a legal port of entry, pushing a hard line on immigration ahead of next week’s congressional elections.

It was not clear whether the plan would pass legal muster. The Immigration and Nationality Act provides that any immigrant in the United States may apply for asylum, regardless of whether he or she entered the country through a designated port of entry.

“Migrants seeking asylum will have to present themselves lawfully at a port of entry,” Trump told reporters at the White House, adding that he would issue an executive order on immigration next week.

“Those who choose to break our laws and enter illegally will no longer be able to use meritless claims to gain automatic admission into our country,” he added.

Trump has ramped up his tough stance on illegal immigration, an issue that appeals to his core supporters, before elections on Tuesday that will determine whether his fellow Republicans keep control of Congress.

In recent days, he has sought to present as a threat to the United States a large group of migrants from Central America who have left poverty and violence at home and are heading slowly through Mexico toward the U.S. border.

“These illegal caravans will not be allowed into the United States and they should turn back now,” Trump said. “We are stopping people at the border. This is an invasion.”

A group of Central Americans who are hoping to apply for asylum, wait at the border on an international bridge between Mexico and the U.S. in Ciudad Juarez, Mexico October 31, 2018. REUTERS/Jose Luis Gonzalez

Republican Senator Chuck Grassley, an ally of the president and head of the powerful Senate Judiciary Committee, echoed some of the president’s strong rhetoric about the caravan in a letter to the secretaries of the Department of Homeland Security and the State Department.

In the letter, Grassley said his office had received information that several members of the caravan had “significant criminal histories” and was seeking information about “potential national security threats that reportedly exist among the members.”

Trump said on Wednesday the United States could send as many as 15,000 troops to the border to confront the migrant caravan, more than twice the number previously disclosed by defense officials.

Republican lawmakers and other Trump supporters have applauded the deployment. But critics argue Trump has manufactured a crisis to drive Republican voters to the polls.

On another immigration front, Trump said this week he would seek to scrap the constitutional right of citizenship for U.S.-born children of noncitizens and illegal immigrants, saying he would issue an executive order on so-called birthright citizenship.

Trump and some other Republicans say the right – granted by the Constitution’s post-Civil War 14th Amendment – creates an incentive for people to enter the country illegally to have children. Critics, including some other Republicans, say the president cannot change the Constitution by fiat. Such an action would virtually guarantee a legal challenge.

(Reporting by Roberta Rampton and Jeff Mason; Additional reporting by Makini Brice and Lisa Lambert; Editing by Peter Cooney)