A Democratic super PAC spent an additional $300,000 in airtime to promote Sen. Sherrod Brown (D-Ohio) this week — yet another sign that his race has become increasingly competitive.

The Majority PAC’s latest buy allows the group’s advertisement to stay on the airwaves a second week. The group has now spent $600,000 to air this spot on Brown’s GOP opponent, Ohio Treasurer Josh Mandel:

https://www.youtube.com/watch?v=qkUfdDWuSy4

Outside groups have been pouring money into the race for months, but Democrats have been outspent by almost a 6-to-1 margin. The latest Majority PAC buy marks the largest investment to boost Brown’s re-election yet by an outside group.

Roll Call rates this race as Leans Democratic.