This cycle’s Nevada Senate race has the possibility of becoming either National Republican Senatorial Committee Chairman John Cornyn’s (Texas) crowning achievement or his white whale.

Those two possibilities were recently underscored by the release last week of yet another poll that showed Majority Leader Harry Reid’s (D) continued vulnerability in the Silver State and by the addition on Wednesday of another second-tier candidate to the GOP primary field.

With approval ratings in the mid-30s, the Senate’s Majority Leader is certainly an enticing target. But Senate Republicans also have a dozen incumbents to protect, crucial open-seat races and juicy pickup opportunities in states such as Illinois and Connecticut.

Republicans lost their top choice in Nevada when Rep. Dean Heller decided to pass on the race, and if no other choice candidate can be found some Republicans are now wondering if going after Reid would be the best use of the NRSC’s resources.

“If they don’t have somebody, why would they make that investment if the candidates are just destined to lose?— one Nevada GOP insider said Wednesday. “I’m not saying that’s the case right now, but that’s how it’s playing out. You have a lot of B-level candidates. I think they have to ask themselves if this is worthwhile.—

It would be no small investment for the NRSC. Reid’s camp has been pushing a $25 million figure when discussing how much the Majority Leader is preparing to spend on the race.

The eventual Republican nominee is not going to have to raise $25 million, “but they are going to have to spend $10 million or $12 million just to be competitive,— said David Damore, a political science professor at the University of Nevada at Las Vegas and a Nevada pundit. “What happens if they blow it here?—

State Sen. Mark Amodei (R), who joined the primary on Wednesday, said he hopes the NRSC will make a decision on who to back in the race by the fall.

“That needs to be done before the primary,— said Amodei, who met with Cornyn in June about the race. “If you don’t have that, in my view you simply won’t win.—

Robert Uithoven, a consultant for Nevada Republican Party Chairwoman Sue Lowden — who announced last week that she was stepping down from her party post to consider the Senate race — said he believes it’s important for the NRSC to weigh in on the primary race by the beginning of 2010.

“I think Nevada is a great investment for the NRSC,— Uithoven said. “It’s not just what they can spend in Nevada but what they can raise, and I think the NRSC is going to be able to raise a lot of money on Harry Reid’s name.—

Some Republicans on Capitol Hill privately admit that Lowden, who has some personal resources and experience in the state Legislature, is probably the best of the more than half-dozen Republican candidates now in the race. For now, the NRSC is staying mum on whom it prefers.

“There are a number of qualified candidates looking at the race, and we’ll let that process play itself out,— NRSC spokesman Brian Walsh said. “Ultimately, this race will be a referendum on Harry Reid. … The simple fact that several candidates or potential candidates are polling ahead of the Senate Majority Leader without the NRSC or their own campaigns spending a dime speaks to just how precarious his political standing is in Nevada.—

The good news for Cornyn, though, is that he doesn’t have to go all in on Nevada just yet.

In some ways the NRSC’s decision not to promote one candidate has kept Reid’s camp from focusing its attacks (and war chest) on any single person in the crowded primary. Meanwhile, Republican strategists love watching Reid’s poor poll numbers suck up as much airtime as possible.

But Reid has spent more than $4 million on his re-election as of June 30, and GOP candidates are already feeling the effects of the Reid machine when it comes to in-state fundraising.

“Nevada donors are too afraid to have their names on a [Republican] fundraising report,— the Nevada insider said. “Whoever the committee gets behind is going to make or break this— primary.