The Republican shot at unseating U.S. Sen. Bill Nelson is slipping away, according to a new Miami Herald/Tampa Bay Times poll.

Nelson leads Republican U.S. Rep. Connie Mack IV 48-40 percent, the new poll shows, a three-point shift in the Democrat’s favor since July.

That’s the good news for Mack, who is losing by double digits in a slew of other recent surveys.

With the exception of an outlier poll from an Orlando firm, Gravis Marketing, Mack has been trailing between eight and 14 points in the last seven statewide polls. The average of those polls, as compiled by Real Clear Politics, lists the spread at eight points, matching the Herald/Times poll.

The percentage of undecided voters, however, remains unusually high at 11 percent with a little more than six weeks to go in the race.

Mack’s decline is even sharper when voters are asked if they have a favorable or unfavorable opinion of him. In July, 30 percent of voters surveyed said they had a favorable view of the four-term U.S. representative from Cape Coral and his unfavorable rating was only 13 percent. Now, Mack is disliked by 33 percent of the voters surveyed and his favorability rating has dropped to 27 percent.

The telephone survey of 800 registered Florida voters — all likely to vote in the Nov. 6 election — was conducted Sept. 17-19 for The Miami Herald, the Tampa Bay Times, El Nuevo Herald, Bay News 9 and Central Florida News 13.

The poll, which included respondents using land-lines and cell phones, was conducted by Mason-Dixon, a nonpartisan Jacksonville-based company. The margin of error is plus or minus 3.5 percentage points.

Mack’s plummet corresponds with Nelson’s aggressive statewide TV campaign that has been pounding the congressman’s hard-partying youth, financial woes, divorce and attendance record in Congress.

“He’s trying to make Mack look worse than he is,’’ said pollster Brad Coker. “So far it’s working.”

Some Republicans fear the gap may now be too wide to close, absent a surge of help from the coattails of Republican presidential candidate Mitt Romney.

“At this point, for Mack to win, Romney would probably have to pull about 53 percent of the vote,” said Jim McLaughlin, a Republican pollster.

Meanwhile, Nelson is acting as if Florida is not a swing state. He has agreed to one debate, Oct. 17, and has not agreed to do a CNN-sponsored debate of swing-state races on Oct. 30.

By contrast, Mack has scheduled a bus tour through North and Central Florida next week, the only region where polls show he is ahead. He is hoping to appeal to his base and weaken Nelson’s pull among conservatives in the region.

Nelson is not entirely out of the woods. He has been heavily targeted by Republicans as one of the vulnerable incumbents they hope to oust in their march toward regaining a majority in the U.S. Senate.

Coker points to Nelson’s low favorability ratings, which have dropped from 36 percent in the July poll to 33 percent now.

“That’s probably due to the fact that people aren’t really crazy about Nelson, but they’re just learning about Mack,’’ Coker said. “It’s still a situation where Nelson isn’t comfortable yet.”

Mack avoided much of the shrapnel from his past during the primary, keeping a low profile, but that also created the opening for Nelson to profile him.