With polling in Kentucky suddenly showing Senate Minority Leader Mitch McConnell (R) in a dead heat with Democratic upstart Bruce Lunsford, Senate Republicans are privately starting to worry their leader could be knocked off and have begun contemplating what their leadership might look like in his absence.

McConnell still retains a significant cash advantage over Lunsford, and Kentucky is a conservative, Republican-leaning state. But a Mason-Dixon Polling and Research survey conducted late last month showed McConnell with 45 percent and Lunsford with 44 percent, while a SurveyUSA poll taken at the same time showed the margin at 49 percent to 46 percent.

Those numbers have so encouraged Democrats that the Democratic Senatorial Campaign Committee this week went up with its first TV ad against McConnell — something that even DSCC Chairman Charles Schumer (N.Y.) acknowledged was unthinkable a few weeks ago.

Schumer said Wednesday that Kentucky and Georgia — where GOP Sen. Saxby Chambliss is in an unexpectedly tough race — are now on his radar as potential pickups given recent polling.

“They’re both tied. Our private polling mirrors the public polling that these are even-steven races. We believe we can win in both of those states. We’re devoting resources to them. In fact, you can go on our Web site. Our first Kentucky ad starts today,” Schumer said.

And with voters nationwide increasingly unhappy with incumbents, particularly of the GOP variety, Republicans for the first time are quietly grappling with the possibility that they could face a 60-vote Democratic majority in the Senate and a potential leadership shake-up in November.

Asked whether Republicans have internally entertained the possibility, a top Republican strategist said: “They are just starting to, just a little bit. You need to prepare a contingency plan for ‘just in case’ scenarios like this one.”

A McConnell defeat, Republicans say, would be salt in the wound for a party staged to suffer significant losses in the House and Senate, and perhaps the White House.

“That would be the worst thing for any of us,” one GOP Senate aide said. “If McConnell loses, it would be bad for the caucus, it would be bad for the leadership team. Regardless if you love him or hate him, he has kept the caucus unified and has done it without beating people up.”

A Republican leadership aide agreed, saying that while he has often differed with the conservative wing of his party, McConnell has been effective in keeping an often disparate Conference together. “There would be a tremendous void in the ability to corral the Conference. There’s no one else out there, like a [former Sen.] Don Nickles who has that ability,” the aide said.

The leadership aide also argued that a McConnell loss would almost certainly be part of a much larger turnover in the Senate, arguing that in addition to losing their leader, Republicans would also be largely powerless to stop legislation in the Senate. “You’re well north of 60 [votes] if McConnell loses,” this source said.

“If McConnell loses, the Republicans would need to create stability immediately,” one GOP Senate strategist said. “They will need to coronate a new leader quickly in order to show strength because it’s going to be a party in disarray.”

Although there are numerous leadership shake-up scenarios, most agree that the current No. 2, Minority Whip Jon Kyl (R-Ariz.), would be the lead candidate to replace him.

“The way Republicans do things generally, there’d have to be a very serious realignment for Jon Kyl not to take the slot,” the leadership aide said.

Kyl has worked his way up through the ranks to amass a long, policy-heavy résumé and understanding of the Senate, its rules and its Members. While a staunch conservative, Kyl also has cut deals in recent years, particularly on dicey topics such as immigration reform.

“He is the top contender for the position,” said another Republican Senate aide.

Kyl might be the heir apparent, but other possible candidates could also emerge, such as Sen. Judd Gregg (R-N.H.), one of the Conference’s respected policy wonks who upped his credibility earlier this month in negotiating a bipartisan economic rescue package.

But Gregg has shied away from elected leadership posts in the past, and many say he is likely to continue as a back-bencher who serves as one of the leadership’s top counselors.

One Republican also noted that Gregg has made few forays outside of budget and spending-related issues. Gregg hasn’t had “to deal with the pressures of managing a Conference. … I don’t know how well that translates into a leadership position,” the Republican said.

Other ambitious Senators could consider running, such as Conference Chairman Lamar Alexander (Tenn.), but most Republican insiders believe Alexander is more likely to run to succeed Kyl as Minority Whip or stay in his current job as the top messenger for the Conference.

Another likely candidate to replace Kyl would be Sen. John Thune (S.D.), now the Chief Deputy Minority Whip. Thune has hinted at running for leadership in the past but opted against it. Many Republicans believe he would strongly consider making a run if the top Whip post came open.

Whether Alexander would run for it as well remains unclear, but he has sought the post before. He ran for it in 2006 against then-Sen. Trent Lott (Miss.), losing by one vote.

The dominoes continue to fall from there. Alexander’s job as Conference chairman — if he were to choose to run for Whip — would have broad appeal as well. Several up-and-comers such as Sens. Richard Burr (N.C.) and Johnny Isakson (Ga.) or Conference Vice Chairman John Cornyn (Texas) could be interested.

Cornyn has started running for chairman of the National Republican Senatorial Committee, however, and many believe he will still pursue that job regardless of McConnell’s fortunes, although at least one Republican speculated that if the party’s ranks are seriously thinned on Election Day, Cornyn’s name could come up as a potential challenger for a higher slot in leadership.

Also considering running for the NRSC next cycle is Sen. Norm Coleman (Minn.), who might look at other options if more than one job is vacant.

That leaves the current NRSC chairman, Sen. John Ensign (Nev.) , who has mounted a bid to serve as the GOP Policy Committee chairman next year. If McConnell lost, most believe Ensign would stick to his plan, but he could also consider running for Conference chairman.

The final seat in the hierarchy, the Conference vice chairmanship, is likely to attract a race. Burr, Isakson and Coleman are considered likely contenders for that post, along with Sens. Lisa Murkowski (Alaska) and Mel Martinez (Fla.).

Another aide said that regardless of how the leadership races shake out, there will be a desire to “more quickly and decisively” project calm and order. That will be particularly true, this staffer said, if the Democrats win the White House on Nov. 4.

“We won’t have time to fight amongst ourselves,” this staffer said. “The Obama administration would likely transition quickly and, by controlling the House, be ready to jam stuff through. Our guys need to be organized and back on the offense immediately after this election.”

Emily Pierce contributed to this report