‘Is this not a massive failure?’ The latest data from the border draws mixed reactions on the Hill.

A decrease in migrant children who arrived alone at the southern border last month gave Homeland Security Secretary Alejandro N. Mayorkas some ammunition to try to persuade senators on Thursday that the Biden administration is making progress containing a surge of migrants.

But April also set a record for the total number of migrants apprehended at the border, and Republicans on the Senate Homeland Security and Governmental Affairs Committee told the secretary that what they called a crisis has only gotten worse.

More than 178,000 migrants were caught last month trying to cross the American border with Mexico, a slight increase over the record-setting numbers seen in March, according to the latest data from Customs and Border Protection, a division of the Homeland Security Department. Most of the migrants were from Central America, fleeing violence, poverty and natural disasters.

Of those, more than 17,000 were migrant children who arrived alone, were apprehended by Border Patrol officers and then transferred to federal shelters where they wait as officials try to place them with family members or other sponsors living inside the United States. The number is down 9 percent from March.

The number of migrants who try to cross the southern border typically increases in the spring months. Both the Obama administration in 2014 and the Trump administration in 2019 saw unusually high numbers of children arriving at the border alone.

But this year has been worse, and the situation at the border has become a partisan flash point, with Republicans contending that migrants are taking advantage of weak Biden administration policies.

“The crisis today is unprecedented, far worse than it was last year, and even substantially worse than 2019, when everyone considered it a crisis,” said Senator Rob Portman of Ohio, the top Republican on the committee.

Mr. Mayorkas and other Biden officials have avoided describing the situation on the southern border as a crisis. And they point to recent successes of transferring the children out of the Border Patrol’s jail-like facilities and into large shelters overseen by the Health and Human Services Department. For example, on March 29, 5,767 migrant children were held in Border Patrol detention facilities for an average of 133 hours, nearly twice as long as what is legally allowed. At the same time, 11,886 children were in shelters run by Health and Human Services.

Those numbers have shifted after the government was able to bring on a network of emergency shelters. As of Wednesday, there were 536 migrant children in Border Patrol custody for an average of 24 hours. And 20,397 children were under the care of Health and Human Services, waiting in government custody to be placed, ideally, with a family member as their cases move slowly through the immigration court system. Many of the children who arrived at the border in 2019 are still in the country waiting to see an immigration judge.

For Republicans, the watchword is “crisis.”

“I see an extraordinary crisis,” Senator Mitt Romney, Republican of Utah, said on Thursday.

“I mean, do you recognize this as an alarming crisis?” he asked Mr. Mayorkas.

Mr. Mayorkas responded, “I look at immigration as a challenge that has been persistent for many, many years,” but was cut off by Mr. Romney who pointed to border data that was on display in the hearing room and said, “Is this not a massive failure?”