LOS ANGELES — President Obama will send up to 1,200 National Guard troops to the Southwest border and seek increased spending on law enforcement there to combat drug smuggling after demands from Republican and Democratic lawmakers that border security be tightened.

The decision was disclosed by a Democratic lawmaker and confirmed by administration officials after Mr. Obama met on Tuesday with Republican senators, several of whom have demanded that troops be placed at the border. The lawmakers learned of the plan after the meeting.

But the move also reflected political pressure in the president’s own party with midterm election campaigns under way and with what is expected to be a tumultuous debate on overhauling immigration law coming up on Capitol Hill.

The issue has pushed Janet Napolitano, the secretary of homeland security, into something of a corner. As governor of Arizona, she demanded that Guard troops be put on the border. But since joining the Obama administration, she has remained noncommittal about the idea, saying as recently as a month ago that other efforts by Mr. Obama had made the border “as secure now as it has ever been.”