The British singer and father-of-eight was moved by a report from US broadcaster CNN that the families had hired a bus to take them more than 1,000 miles from Baton Rouge, Louisiana, to Washington DC.
The effort to show their disdain at the Republican party’s proposals had set them back around 30,000 dollars (£23,000) but, at the time, they had raised only 7,000 dollars (£5,000).
It emerged on Thursday that Stewart, 72, decided to send them a cheque to cover their deficit after viewing the report.
According to the broadcaster, he wrote to his manager: “I’ve just seen something on CNN that’s heart-breaking. It was a group of families with severely disabled children who are driving to Washington to confront about health care cuts.
“See if you can find out who they are. I’d like to help in some way.”
Angela Lorio, co-organiser of the group Trach Mommas of Louisiana, praised the singer in a video diary.
“So I just deposited the biggest cheque of my life, and it couldn’t be from a more amazing person,” she said.
“Medicaid being cut, it’s not just numbers. And to have a person like Rod Stewart be so touched that he would do something like this to pay for the rest of our bus trip, it just blows my mind - it really, really blows my mind.”
The broadcast on July 10 had highlighted the impact of the Republican’s proposed cuts to Medicaid, funding that helps poorer families.
Lorio said her young son John Paul, who was born prematurely with severe health problems, would die if the cuts were approved.