Club for Growth Action drew a four-fold increase in fundraising last month as attack ads against Donald Trump intensified.

The PAC, one of the frontrunner’s earliest and most prominent critics, had its biggest haul of the 2016 election cycle, raising $4 million in February, up from less than $1 million a month earlier and just $162,416 in December, according to its filing with the Federal Election Commission today. The group also spent $4.7 million, 59 percent of which was directed against Trump.