President Barack Obama skewered Republicans Monday for failing to "walk the walk" on their promises to middle-class Americans.

Speaking at a Labor Day union rally in Boston, the president called out opponents of his economic policies and Republican presidential candidates who claim to be serving the middle class but then "do things that hurt the middle class." 

"You just wait, you look up at the sky and prosperity will come raining down on us from the top of whatever high-rise in New York City," he joked. "But that's not how the economy works." 

He said Republicans have been "wrecking the economy for a long, long time."

"These folks are pretty stubborn. I will give them credit -- they don’t let facts or evidence get in the way," he said.

Obama took shots at several Republican presidential hopefuls, saying he was grateful not to be on the ballot this time around. He didn't name names, but gave examples of anti-union remarks known to be made by Carly Fiorina, Wisconsin Gov. Scott Walker and New Jersey Gov. Chris Christie.

"You had one that blamed unions for the women's pay gap," Obama said of Fiorina. "Think about that. So, suddenly, if there were no unions, all women would be paid equal?"

He hit Christie for saying a teachers union deserves "a punch in the face" -- "Really? Tell me how you really feel," the president said. He also gawked at Walker for "bragging about how he destroyed collective bargaining rights in his state and says that busting unions prepares him to fight [the Islamic State]."

Obama also pitched his newly signed executive order on paid sick leave in the address. The order requires federal contractors to grant their employees up to seven sick days annually, a move the White House says would affect about 300,000 people.

"It helps with recruitment and retention," the president told attendees of the rally on Monday.

"Now, unfortunately, only Congress has the power to give this security to all Americans," Obama said. "But where I can act, I will. And, by the way, I just did."

Read more on the executive order here.