Both parties are fighting this week for senior voters in the special election to replace former Rep. Bill Young (R-Fla.) this week.

Sen. Marco RubioMarco Antonio RubioNew fears over Chinese espionage grip Washington Rubio heckled by protestors outside immigration detention facility Bill to protect work licenses of student loan debtors is welcome development MORE (R-Fla.) endorsed Republican David Jolly, campaigning with him just as a Democratic outside group knocked him on Social Security in a new ad.

The senator appeared with Jolly Monday at a senior center in the district and touted the GOP nominee's commitment to the elderly.

“I know David will fight to stop Obamacare from hurting seniors and families in Pinellas County and throughout our state,” Rubio said in a statement announcing his endorsement.

Rubio cited Jolly’s commitment to combatting ObamaCare as the reason behind his endorsement, which he said has had a “devastating impact” on the district.

Former Florida Gov. Jeb Bush (R) and Sen. John McCainJohn Sidney McCainMulvaney aims to cement CFPB legacy by ensuring successor's confirmation Trump mocks McCain at Nevada rally Don’t disrespect McCain by torpedoing his clean National Defense Authorization Act MORE (R-Ariz.) have also endorsed Jolly. Democrat Alex Sink, meanwhile, has gained the support for Vice President Joe BidenJoseph (Joe) Robinette BidenBiden: Trump family separation policy could make the US a pariah Elizabeth Warren can unify Democrats and take back the White House Giuliani doubles down on Biden comments: 'I meant that he’s dumb' MORE, who hosted a fundraiser for her.

Rubio’s campaign stop and the emphasis on seniors and retirees — who make up a significant portion of the voting population in Florida’s 13th district, one of the oldest in the nation — came as Jolly faced renewed attacks from a Democratic outside group focused on his career as a lobbyist.

The new ad, from House Majority PAC, charges that Jolly “lobbied for a special interest that wanted to privatize Social Security,” a proposal that opponents say would reduce benefits and undermine the solvency of the program.

Jolly’s lobbying career has been a focal point of Democratic attacks throughout the campaign. They point to a lobbying disclosure report that reveals Jolly addressed Social Security reform with Rep. Paul RyanPaul Davis RyanDOJ delivers Russia probe documents to Congress Laura Ingraham: George Will is ‘sad and petty’ for urging votes against GOP Seth Rogen: I told Paul Ryan I hate his policies in front of his kids MORE (R-Wis.) during his work for a conservative group whose CEO expressed support for privatizing the program.

Jolly has said he didn’t lobby on Social Security, and that he was simply “overcomplying” with disclosure requirements because the issue came up briefly in conversations with Ryan.

The attack is the latest in a volley from outside groups affiliated with both parties meant to sway a race that both public and private polling has shown to be close.

Though the district became more competitive for Democrats with former Rep. Bill Young’s (R-Fla.) passing, the unpopularity of ObamaCare and the president himself in the district make it a tough fight for Sink.

The stakes are high for Democrats, however — the race is seen by many as a bellwether of what the party can expect come November.