Sen. Marco Rubio spoke up for Mitt Romney onstage at the Republican National Convention in Tampa, and he's doing it again on TV airwaves.

The Romney campaign is now airing an ad in Florida that stars Rubio, who speaks straight to camera about Medicare, a hotly debated issue of particular concern to seniors, of which Florida has many.

The ad aired in Miami just after 5 a.m. today, according to Kantar Media's Campaign Media Analysis Group, which tracks campaign ads.

In it, Rubio touts the Romney-Ryan Medicare-overhaul plan, mentioning his mother. Rubio tells viewers:

Mitt Romney and Paul Ryan get it. Medicare is going broke; that's not politics, it's math. Anyone who wants to leave Medicare like it is is for letting it go bankrupt. My mother's 81 and depends on Medicare. We can save Medicare without changing hers, but only if younger Americans accept that our Medicare will be different than our parents' when we retire, in 30 years. But after all they did for us, isn't that the least we can do?

Rubio, 41, spoke before Romney on the final night of the Republican National Convention in Tampa Aug. 30.

Both parties have attacked each other on Medicare repeatedly during the campaign. Romney has hit President Obama for Medicare cuts (or, as Democrats call them, savings) in the president's signature health-care law; Obama has hit Romney and vice presidential nominee Paul Ryan on Ryan's plan to effectively "voucherize" Medicare, replacing the current fee-for-service model with "premium-support payments" to subsidize private insurance plans for seniors.

In August, Paul Ryan touted his Medicare plan while campaigning alongside his mother at The Villages, a Florida retirement community.

According to 2008 exit polls, 22 percent of Florida's presidential-election voters were 65 or older, and 49 percent were 50 or older.