Donald Trump supporters are on high alert after hearing about an all-out ad blitz their Democratic rivals are about to unleash on the GOP's presumptive nominee.

"This is not a drill. Hillary's campaign finally has Bernie behind them and they are going full steam ahead to take Donald Trump down," a Saturday message from the Great America Political Action Committee to Trump supporters reads.

The message was in response to presumptive Democratic nominee Hillary Clinton announcing a $27 million campaign of attack ads targeting Trump in key battleground states.

"The Clinton campaign is unleashing their millions against Trump on the airwaves," the message reads, asking for donations up to $1000 to hit back at Clinton.