The NRA's recent slump in spending comes as the gun rights group is experiencing an unprecedented level of scrutiny owing to a widening financial and governance scandal.
The spread has since been characterized as "incitement" and a "call for violence."
Giffords celebrates her 47th birthday Thursday after surviving a 2011 assassination attempt.
Giffords and other Democrats met on Capitol Hill Wednesday, just days after President Trump addressed thousands of NRA members.
Clinton will campaign Monday night in Cincinnati alongside gun-violence survivor Gabrielle Giffords, a former U.S. representative.
The national tour will reach at last 43 cities across 14 states in six weeks and end on Election Day.
All official EU referendum campaigning has been suspended until Saturday.
The presidential candidate called for tighter gun restrictions after two journalists were killed during a live broadcast in Virginia.