Now, third-party groups, such as the Freedom PAC, which received $1 million from casino magnate Sheldon Adelson, are coming to Mack’s aid and financing a two-week pro-Mack ad that shows excerpts of his speech at the Republican National Convention.

Another ad, which Mack ran last week, is being paid for by the National Republican Senatorial Committee. In it, Mack attempts to tie Nelson with President Barack Obama, saying they are “lockstep liberals.”

The liberal line was used successfully by Mack’s father, the former U.S. senator of the same name, when he ran in 1988. The elder Mack attacked his opponent, Buddy MacKay, with the line: “Hey Buddy, You’re a Liberal.”

Nelson, the last of Florida’s cracker-style Democrats who has held elected office since 1972, has kept his distance from his party’s liberal wing. He built his career appealing to moderate and conservative voters, even staying away from the Democratic National Convention, except for a brief appearance to raise money.

His supporters say that is part of the reason Nelson has retained his seat despite challenges from heavily financed Republicans. It may also explain why, according to the Herald/Times poll, he gets 53 percent of the independent vote, compared to 34 percent by Mack.

Pollsters say that while enthusiasm isn’t deep for Nelson among Democrats, neither is it this year among Republicans for Mack.

Mack should be able to rely on votes from people like John Wiegner, a 64-year-old retired U.S. Special Forces colonel from Valrico. Wiegner is a registered Republican and no fan of Nelson — but he is also lukewarm about Mack.

“The only thing going for him is his great-grandfather,” said Wiegner, who grew up in Allentown, Pa., and rooted for the Philadelphia Athletics, which were managed by Mack’s great-grandfather, Cornelius McGillicuddy Sr.. who was also known as Connie Mack, from 1901 to 1950. “But he’s not bringing anything else to the table. He’s not giving me any reason to vote for him.”

Wiegner said one turn-off is the tenor of the ads both candidates have aired.

“They’re both so negative,” he said. “I abhor not voting for a candidate, so I won’t, but Mack isn’t making it easy for me.”

Miami Herald staff writer Marc Caputo and Tampa Bay Times reporters Michael Van Sickler and Katie Sanders contributed to this report. Mary Ellen Klas can be reached at meklas@MiamiHerald.com and on Twitter @MaryEllenKlas