Correction Appended

At a press conference just before the August recess National Republican Senatorial Committee Chairman John Cornyn (Texas) boasted of his committee’s abilities to turn around what was widely viewed as a tough election map for the GOP at the beginning of the cycle.

But 15 months from the midterm elections, several big holes still remain when it comes to Senate Republican recruiting efforts. Whether the NRSC can plug those holes after Congress returns in September could be the difference between Republicans fighting to hold onto the territory they already have in 2010 and the NRSC actually mounting a serious offense against incumbent Democrats next year.

The decision on Tuesday by Rep. Dean Heller (R-Nev.) to pass on a race next year against Senate Majority Leader Harry Reid (D-Nev.) underscores the work that Republicans have left to do when they get back from recess.

Since the beginning of the 2010 cycle Republicans have been excited about giving Reid a real race next year, but they haven’t found a top-tier challenger to rally behind. And while early polling numbers continue to show that Reid may well be vulnerable, the Majority Leader has spent the past eight months banking millions for his re-election effort while the names of top GOP recruits — including Lt. Gov. Brian Krolicki, former Rep. Jon Porter and now Heller — have come and gone.

While Republicans insist that they’ll eventually have a top-tier candidate in the race, the longer that recruiting process takes, the more time Reid will have to pad his war chest. In addition to Nevada, Republicans also have candidate recruiting holes to fill in Arkansas and Colorado, and the NRSC also needs to move a pair of highly-touted potential recruits off the fence in California and Delaware.

That list of possible targets should sound familiar to anyone who has been tracking Cornyn since he took over the reins of the NRSC.

In a speech at the Conservative Political Action Conference in late February, Cornyn said Senate Republicans could face a “very promising— landscape in 2010 if they could take advantage of “opportunities in unexpected places.—

In that speech Cornyn named New York, Connecticut, Delaware, Illinois, Arkansas, Colorado, Nevada and California as states where those opportunities exist.

New York continues to be a long shot for Republicans, but Cornyn has found a strong candidate in Illinois with Rep. Mark Kirk (R), who faces a primary but is widely expected to be the GOP nominee.

Republicans face a more complicated primary field in Connecticut, but former Rep. Rob Simmons (R) is the party’s preferred nominee, and he would be a formidable opponent against Sen. Chris Dodd (D). Public polling has shown Dodd trailing some of his potential GOP opponents and the Senate Banking, Housing and Urban Affairs chairman is viewed as the most vulnerable Democrat up for re-election next year.

In Delaware, Republicans are keeping their fingers crossed that Rep. Mike Castle (R) will throw his hat into the open Senate race.

Castle, a former governor, is the state’s most popular Republican and the only candidate who could give Democrats a real run in the First State. A number of Republican Senators have already tried to convince Castle to make the jump to the other side of the Capitol, and the Congressman has said he’s more likely to run for Senate or retire than he is to seek re-election. The NRSC is just hoping he doesn’t pick retirement.

In California the NRSC also has a top potential recruit in former Hewlett Packard CEO Carly Fiorina (R). She’s well-known and could raise buckets of money for a race against Sen. Barbara Boxer (D), but she hasn’t made a final decision yet on the race. Colorado and Arkansas remain the two biggest question marks for the NRSC as September approaches.

Despite GOP dominance in the state when it comes to presidential elections, the Republican bench in Arkansas is weak at best. The list of Republicans who have filed for the race already certainly doesn’t have Sen. Blanche Lincoln (D) supporters worried, and the Senator’s $3.2 million in the bank is a daunting figure to any Republican thinking of challenging her.

Now in her second term, Lincoln has taken some flak on the left for her wavering support of the Employee Free Choice Act but with the bill now on the back burner, Lincoln may not have to worry about the repercussions that a vote on the legislation could have on her electoral prospects this cycle.

When he was appointed in January to replace Sen. Ken Salazar (D-Colo.), Sen. Michael Bennet (D-Colo.) seemed like a juicy target for Republicans. Bennet, who previously served as superintendent of public schools in Denver, was not well-known outside the city, and it was unclear whether he could raise the money to compete in a statewide race. Eight months later Bennet seems to have put chatter about a possible primary behind him while Colorado Republicans have yet to rally behind a single candidate.

Aurora City Councilman Ryan Frazier (R) and Weld County District Attorney Ken Buck (R) are in the race, as is businessman Cleve Tidwell. Meanwhile former state Sen. Tom Wiens (R) has filed an exploratory committee. Outside that group, former Rep. Bob Beauprez, and radio host Dan Caplis are other GOP possibilities. Despite the many names currently being mentioned, none has yet to be championed by the national party and insiders admit that the field could continue to grow.

For their part, Senate Democrats — who have a 60-seat majority in the chamber — have fewer holes to fill when it comes to the recruitment game.

North Carolina remains the most glaring question mark for the party. Early polling has shown that Sen. Richard Burr (R) could be vulnerable in 2010 but a handful of House Members and statewide elected officials have all passed on the race.

The latest name being tossed around in Democratic circles is North Carolina Secretary of State Elaine Marshall (D), who is seriously considering taking on Burr.

Democrats also don’t officially have a top-tier candidate in Louisiana, where Sen. David Vitter (R) is facing voters for the first time since being caught up in the D.C. Madam prostitution scandal two years ago. But party insiders remain confident that Rep. Charlie Melancon (D-La.) will soon fill that hole and give Vitter a real race next year.

Correction: Aug. 13, 2009

The article incorrectly stated that Colorado Senate Minority Leader Josh Penry was a possible candidate for the U.S. Senate. He is running for governor.