Fencing has been installed ahead of Harris' speech at the Ellipse in Washington.
A new permit issued to the Harris campaign says up to 40,000 people are expected to attend.
The event, for which gates open at 3 p.m. ET, has been advertised to Harris supporters with push alerts, emails and online ads.
The Secret Service said in a statement to ABC News that it is "working closely with our federal and local partners to ensure the safety and security of Vice President Harris and all attendees at the campaign event on Tuesday at the White House Ellipse.
Several protest groups have announced plans to protest Harris' speech, including a large group of anti-war and pro-Palestinian protesters from several well known Washington-based groups.