Republicans Sen. Ted Cruz, R-Texas, and Rep. Elise Stefanik, R-N.Y., sent a letter to the Biden administration urging officials to enforce federal immigration law to combat a migrant caravan that is approaching the southern border.

"We are deeply concerned about what the Department of Homeland Security (DHS) is doing to prevent a similar situation at our southern border, especially when Customs and Border Protection (CBP) field agents are already overworked, undermanned and under-supplied as a result of the influx of migrants this year," Cruz, Stefanik, and over a dozen other Republicans wrote in a letter to Homeland Security Secretary Alejandro Mayorkas.

U.S. Senator Ted Cruz (R-TX) speaks during a television interview in response to U.S. President Joe Biden's first address to a joint session of the U.S. Congress, at the U.S. Capitol in Washington, U.S., April 28, 2021.

U.S. Senator Ted Cruz (R-TX) speaks during a television interview in response to U.S. President Joe Biden's first address to a joint session of the U.S. Congress, at the U.S. Capitol in Washington, U.S., April 28, 2021. (REUTERS/Erin Scott/File Photo)

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"Additionally, this caravan comes only weeks after ten thousand migrants flooded Del Rio, Texas, overwhelming border personnel and later gaining entry into the United States," the letter added. "While the strategy DHS released on September 18, 2021, stated that migrants ‘who cannot be expelled under Title 42 and do not have a legal basis to remain will be placed in expedited removal proceedings,’ this did not occur in Del Rio. In fact, Senior Executive Branch officials stated that migrants were released into the United States on a ‘very, very large scale.’ We believe this incentivized the recent caravan to journey to the United States."

In the letter, the Republicans urged the Biden administration to "enforce our immigration laws as we do any other laws and ensure migrants in this caravan with nonexistent asylum claims will not be released into the United States."

Oct 2021: A woman making up one of the thousands of migrants coming north as part of the migrant caravan.

Oct 2021: A woman making up one of the thousands of migrants coming north as part of the migrant caravan.

A massive migrant caravan organized via QR code left the town of Huehuetán in the south of Mexico in late October on the third day of their long trek toward the U.S. southern border.

The caravan, made up of thousands of mostly Central Americans, South Americans and Haitians, is the largest and most organized of its kind this year, with participants registering to join via QR code starting on Oct. 15.

"Tell Biden we are coming," one migrant named William from El Salvador told Fox News.

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Video footage captured by Fox News showed thousands of migrants, including small children being pushed in strollers, walking north about 20 miles north of Tapachula. One migrant carried a large wooden cross at the front of the caravan while others carried American flags and signs with President Biden’s name.

"The caravan is like a magnet, it goes sucking up people, migrants who had been in the towns [of coastal Chiapas] are joining," said Irineo Mújica, an immigration activist with the organization People without Borders.

One of them was Bayron Zavala, a Nicaraguan migrant, who upon hearing that the caravan was advancing slowly, got on a bicycle and caught up with them in Huixtla. He said he would walk with them "as far as God gives us the strength ... if possible, continue to the United States."

Without any issue, the migrants passed a customs, immigration and military checkpoint where authorities typically seize drugs and look for human smugglers.

Though still significantly smaller than caravans in 2018 and 2019, this is the biggest group moving through southern Mexico since the pandemic started early last year. In January, a caravan left Honduras, but authorities in Guatemala broke it up.

Other groups that have walked out of Tapachula this year have numbered in the hundreds. All were dissolved by Mexican authorities, sometimes with excessive force. Those groups were composed mostly of Haitian migrants. This caravan is primarily made up of Central Americans.

Representative Elise Stefanik, a Republican from New York, speaks during a news conference before a hearing for the Select Committee to Investigate the January 6th Attack on the U.S. Capitol in Washington, D.C., U.S., on Tuesday, July 27, 2021. 

Representative Elise Stefanik, a Republican from New York, speaks during a news conference before a hearing for the Select Committee to Investigate the January 6th Attack on the U.S. Capitol in Washington, D.C., U.S., on Tuesday, July 27, 2021.  (Al Drago/Bloomberg via Getty Images)

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The National Guard has not tried to intervene since it attempted to keep the migrants from Tapachula on Saturday. There were scuffles, and a child was injured.

Foreign Affairs Secretary Marcelo Ebrard said Monday the government would act "prudently," respecting the law and human rights.

Mexico has deployed thousands of soldiers, police and immigration agents in the south, and in recent years, no large groups have made it out of the states bordering Guatemala.

The Associated Press and Fox News’ Griff Jenkins and Jessica Chasmar contributed to this report