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 RubioSenate GOP to Obama: Stop issuing new rules Juan Williams: McConnell won big by blocking Obama The ignored question: What does the future Republican Party look like? 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 McCainFree speech is a right, not a political weapon The trouble with Rex Tillerson   Senate: Act now to save Ukraine MORE (R-Ariz.) have also endorsed Jolly. Democrat Alex Sink, meanwhile, has gained the support for Vice President Joe BidenJoe BidenSenate names part of Cures bill after Beau Biden Overnight Finance: Trump adviser softens tone on NAFTA | Funding bill to be released Tuesday | GOP leader won't back Trump tariff plan Cures bill clears first Senate hurdle 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 RyanDr. Price’s first 100 days: What to kill and what to keep Medicare looms over Trump-Ryan alliance Trump allies warn: No compromise on immigration 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.