WASHINGTON — President Trump has begun a new push for legislation to crack down on illegal immigration and make it more difficult to obtain refuge in the United States, White House officials said Monday, arguing that lax laws have drawn a flood of migrants to the country’s borders.

The proposals include toughening laws to make it more difficult to apply for or be granted asylum in the United States, stripping protections for children arriving illegally without their parents so they can be turned back at the border or quickly removed, and allowing families to be detained for longer periods while they await decisions from immigration authorities about their fates.

While the steps have long been advocated by Mr. Trump’s hard-line aides, including Stephen Miller, his senior policy adviser, focusing on them now opens a new front in the president’s push for immigration restrictions.

There is no evidence of an overall increase in people crossing the southern border with Mexico illegally — in fact, the president has often boasted that the number has dropped since he took office, in part because of his administration’s policies. But Mr. Trump’s renewed legislative effort is a return to the hard-line anti-immigration themes that animated his campaign and much of his presidency at a time when he is facing a backlash from some disenchanted conservatives for signing a $1.3 trillion spending bill that did not include funding for his border wall.