As Congress prepares to head home for the July Fourth recess, the 2008 electoral map shows a Senate playing field that favors Democrats — based on sheer numbers — but also has more questions than answers, less than a year and a half before Election Day.

While Democrats remain confident they will not see any retirements this cycle, Republicans still have at least two, and possibly three, big question marks that have the potential to create top-tier, open-seat contests.

Meanwhile, recruiting success on both sides of the aisle has been minimal at this point, as both parties have been unable to coax top-tier challengers into races against the Senators believed to be most vulnerable in 2008.

The recruitment burden falls heaviest on Democrats, who are only defending 12 seats compared to the 22 seats the GOP has to protect. Also, there were more Republicans viewed as vulnerable heading into the election cycle.

To this point, Democrats’ biggest success has been the wooing of Rep. Tom Allen (D-Maine) to run against Sen. Susan Collins (R), a moderate, in what will be a hotly contested race next fall.

Maine, Minnesota and Colorado are the most fully formed races to date and all are expected to be competitive showdowns. Democrats are viewed as having the early advantage in Colorado, currently the only open-seat race, where Rep. Mark Udall (D-Colo.) will face off against former Rep. Bob Schaffer (R).

In Minnesota, comedian and liberal talk show host Al Franken and attorney Mike Ciresi are squaring off for the Democratic nomination to face Sen. Norm Coleman (R-Minn.) next year. However neither Democrat is viewed as the strongest possible challenger to Coleman, who bested both men by more than 20 points in a recent independent poll.

Two of the biggest recruiting question marks remaining for Democrats are in Oregon and New Hampshire where Sens. Gordon Smith and John Sununu, respectively, began the cycle pegged as two of the most vulnerable Republicans in the chamber. Both represent states that have trending away from the GOP.

Democratic insiders now believe former New Hampshire Gov. Jeanne Shaheen (D) more likely than not will enter the race against Sununu, but it has been difficult to gauge her current thinking about a rematch. Several Democrats already are in the Senate race, including Portsmouth Mayor Steve Marchand and Katrina Swett, but Shaheen is the preferred candidate of party leaders. Shaheen lost to Sununu in 2002, but last year both of the Granite State’s Republican Congressmen were swept out of office.

In Oregon, Democrats remain confident they will have a top-flight challenger to Smith though they are mum on who it might be. Reps. Peter DeFazio and Earl Blumenauer are among the Democrats who turned down the opportunity to run.

Republicans, meanwhile, have yet to find top-flight challengers to the two Democratic Senators who looked to be the most vulnerable heading into the 2008 cycle.

Sen. Tim Johnson (D-S.D.) has not returned to the Senate this Congress after suffering brain trauma late last year. Democrats insist he is running again and Republicans are in a precarious position as they wait to find out if that is true. Johnson is raising money and expected to return to the Senate later this year.

South Dakota Gov. Mike Rounds (R) is the candidate most often mentioned but it remains to be seen if he — or any Republican for that matter — decides to run against Johnson. If there is an open-seat race, Rep. Stephanie Herseth Sandlin would be Democrats’ first choice but Republicans would be favored to pick up the seat in a presidential election year.

In Louisiana, Sen. Mary Landrieu was believed to be the most vulnerable Democrat at the start of this year, but there has been little movement among Republicans to step up and challenge her.

State Treasurer John Kennedy (D) continues to float the prospect of switching parties and running and Republicans also mention the possibility of Louisiana Secretary of State Jay Dardenne (R) running.

It’s possible that the contest won’t get sorted out until after the state’s gubernatorial election this November, which is good news for Landrieu. Rep. Bobby Jindal (R-La.) is the leading contender to succeed retiring Gov. Kathleen Blanco (D) though he would face heavy pressure to mount a Senate bid should he lose the state contest this fall.

Another red-state Democrat, Sen. Max Baucus (Mont.), appears to be on solid ground in his re-election bid as it doesn’t look like Rep. Denny Rehberg (R-Mont.) is preparing to run.

Elsewhere, Republicans look to have few opportunities to play offense and from a financial standpoint they may have to devote most of their resources to defending incumbents. At the end of May, the Democratic Senatorial Campaign Committee had $10 million more in the bank than the National Republican Senatorial Committee.

Retirement issues also could plague the GOP, with the potential for blockbuster races to emerge in Virginia and Nebraska at the top of the list.

Sen. John Warner (R-Va.) is expected to announce his re-election plans in September, though observers are increasingly pessimistic about the chances he will run for a sixth term.

If Warner does decide to leave, expect Rep. Tom Davis (R-Va.) to be out of the gate and far ahead of most other GOP contenders. He has been raising money, traveling the state and plotting a statewide strategy for months.

But it’s unlikely that Davis, who is socially moderate and from Northern Virginia, will get a free pass in the primary. Former Gov. Jim Gilmore (R), who currently is running for president, or another Member of the Virginia Congressional delegation may run.

Democratic hopes rest solely with former Gov. Mark Warner.

While Virginia has voted for the Republican in every presidential election since 1968, the state’s changing demographics and Mark Warner’s popularity would make it a difficult seat to hold.

Republican prospects of holding onto an open seat in Nebraska look better, but Democrats also hope to force a competitive race there if Sen. Chuck Hagel (R-Neb.) decides to forgo re-election.

Hagel currently faces a bruising primary with state Attorney General Jon Bruning (R). If Hagel gets out, Secretary of Agriculture Mike Johanns, a former Nebraska governor, may enter the race.

While former Sen. Bob Kerrey (D-Neb.) has made some noise about running, Democrats believe it’s more likely that Omaha Mayor Mike Fahey would get in.

Elsewhere, both parties are still keeping an eye on New Mexico, where Sen. Pete Domenici (R) continues to say he will run again.

If Domenici runs, it isn’t likely he’ll face much of a contest even after the negative press surrounding his involvement in the ousted U.S. attorneys controversy. But if the seat opens up, the Land of Enchantment likely would see one of the most competitive races of the cycle.

There are a handful of other races Democrats would like to make competitive, but their prospects for doing so at this point still remain largely up in the air.

In Alaska, Sen. Ted Stevens (R) faces increased scrutiny in the wake of a federal corruption investigation back home that has targeted his son and close associates. Democrats are trying to recruit Anchorage Mayor Mark Begich into the race.

Democrats also are hopeful they can put one or more heavily Republican states in play by recruiting challengers to Sens. John Cornyn (Texas), Elizabeth Dole (N.C.) and Mitch McConnell (Ky.), the Minority Leader.

The DSCC has released polling that showed all three are potentially vulnerable but Texas is the only one of the three where Democrats have an announced candidate.

Personal injury attorney Michael Watts recently entered the race against Cornyn and put $3.8 million of his own money into the effort.

Still, the chances for Democrats’ success in traditionally Republican territory in a presidential year remain debatable.