Sen. Rand Paul (R-Kentucky). Reuters Presidential candidate and Sen. Rand Paul (R-Kentucky) blasted former Secretary of State Hillary Clinton on Twitter after she questioned the economic model of the so-called "gig" economy in a Monday speech.

Clinton, the Democratic front-runner in 2016, said the "'on demand' or so-called 'gig economy'" needs to be examined. She didn't mention the companies by name, but earlier reports suggested she would take aim at firms like Uber and Airbnb.

"Meanwhile, many Americans are making extra money renting out a spare room, designing websites, selling products they design themselves at home, or even driving their own car," Clinton said, according to her campaign transcript.

She added that this new economic model "is creating exciting opportunities and unleashing innovation but it's also raising hard questions about workplace protections and what a good job will look like in the future."

In a series of tweets after the speech, Paul reminded his supporters that Clinton is unlikely to be found hailing an Uber anytime soon. Clinton — also a former first lady, presidential candidate, and US senator — has a Secret Service detail and recently said she hasn't driven her own car since 1996.

Here's Paul's Twitter criticism:

—Dr. Rand Paul (@RandPaul) July 13, 2015

—Dr. Rand Paul (@RandPaul) July 13, 2015