The recent Senate decisions by four Republicans — Colorado Sen. Ben Nighthorse Campbell, former South Carolina Gov. David Beasley and former Reps. Tom Coburn of Oklahoma and John Thune of South Dakota — have scrambled the Senate outlook for November. [IMGCAP(1)]

Each party has one strong takeover opportunity (Georgia for the Republicans and Illinois for the Democrats). The Democrats now have three other good targets (Alaska, Oklahoma and Colorado), while the GOP has five equally attractive takeover targets (both Carolinas, Florida, Louisiana and South Dakota).

Democrats have one of the better recruiting classes that I have seen. But they are running in some very inhospitable places. And having Sen. John Kerry of Massachusetts leading the Democratic ticket isn’t likely to help the party’s Senate candidates in places such as Oklahoma, South Carolina, South Dakota or Alaska.

On the basis of numbers, the Republicans have more top-tier opportunities than do the Democrats. But opportunities don’t always translate into pickups.

The Democrats seem to have better long-shot opportunities. But while takeovers are more likely in Missouri, Kentucky and Pennsylvania than in Washington state, California or Wisconsin, they aren’t, at this point, close to being likely in any of those states.

Where has the Senate landscape changed the most? Clearly in Colorado and South Dakota.

Campbell’s retirement puts another state in play, an important development for the Democrats. They already have one candidate in the race, wealthy businessman Rutt Bridges, who promises to put millions of his own dollars behind his candidacy, and they could have another, if and when Rep. Mark Udall (Colo.) runs.

Whatever his weaknesses, Campbell was not about to be defeated this year. Without him, the GOP’s hold on the seat is uncertain. Gov. Bill Owens would be a formidable candidate, but he has family problems. His interest in the White House also could affect his decision on a Senate run.

Retiring Rep. Scott McInnis (R) has been itching to run for the Senate, and I expect that he’d be a good candidate. Rep. Tom Tancredo (R) is also interested, but his statewide general election appeal is very much in doubt.

In any case, Colorado moves from being in the GOP column to being a tossup — a significant development.

Former GOP Rep. John Thune’s decision to challenge Senate Minority Leader Tom Daschle (D) has GOP strategists breathing a sigh of relief. Thune, who lost by 524 votes to Sen. Tim Johnson (D) in 2002, remains a terrific candidate. Daschle is more charismatic than his colleague Johnson, but he lacks the clout he had as the Senate’s Majority Leader.

Polling shows the race competitive even though Daschle has been running TV ads for months and spent millions last year. Thune, who believes that he has bolstered his campaign team, looks headed for another squeaker.

It isn’t entirely clear how Beasley’s entry has changed the outlook in South Carolina. The former governor was defeated in his run for re-election, and he still carries some baggage from his administration and his last run. But Democratic nominee Inez Tenenbaum’s problem remains the same as it has been: She is a Democrat in a Republican state, and as the state’s top education official, she’ll be forced to defend the state’s performance in education, which could keep her on the defensive.

Oklahoma remains a question mark for the Republicans, but it isn’t because of Coburn’s entry. Indeed, it’s possible that the former Congressman is the best candidate the Republicans could run in the general election. I’m not sure yet.

Democrats have two things going for them in Oklahoma. First, Rep. Brad Carson is a strong candidate. He has a moderate record and is likely to be an aggressive campaigner. I believe that he has been positioning himself for this race for a long time, and he is as politically savvy as you can get.

Second, Oklahoma Republicans appear to be bitterly divided. I’m not talking about the long-standing bitterness between Oklahoma City and Tulsa, but rather the insider-versus-outsider fissure in the party.

Virtually every establishment Republican with a pulse is backing Kirk Humphreys, who until recently was mayor of Oklahoma City. But there has been a growing rift in the state party, with a considerable number of Republicans showing signs of rebellion against the insiders who are in control.

If state Republicans can’t find a candidate behind whom they can unite, they would give Carson just the opening he would need. He could appeal to rural conservatives who are suspicious of Humphreys or Oklahoma City Republicans who are uncomfortable with Coburn’s religiosity and social agenda. Either way, the Republicans have problems.

Democratic buzz that Nancy Farmer is an imminent threat to Sen. Kit Bond (Mo.) seems premature, and the Democratic Senatorial Campaign Committee’s recent press release crowing that Bill Jones’ victory in the California GOP Senate primary somehow changes the outlook in the state is simply goofy.

The key question for the cycle remains whether Democratic Senate candidates in conservative and generally Republican states can win. We still don’t know the answer to that, but we know that the Republicans in most of the states, if they are united, begin with considerable advantages.

Stuart Rothenberg is editor of the Rothenberg Political Report.