WASHINGTON -- Senate Republican campaign chief John Cornyn of Texas was riding high after gaining seven seats during his first stint as the election boss two years ago, but the volatile 2012 cycle has produced so many bumps in the road that he’s had to change course.

After some unexpected setbacks this year, such as an unscripted nominee in – at the time – near-solid Missouri, and suddenly competitive races in states once thought to be safe, Cornyn is now fighting harder than many had anticipated to deliver a Republican majority for the U.S. Senate.

With less than a month to go before the election, Cornyn, head of the National Republican Senatorial Committee, said in an interview that he remains confident his party will succeed, but “it’s been more challenging than anybody ever thought. . . . It’s a little bit harder and in a little bit less of a straight line.”

Republicans hold 47 seats. Cornyn needs to win just four.

The numbers would appear to be in his favor. Republicans only have to defend 10 Senate seats, while his Democratic counterpart, Sen. Patty Murray of Washington, has to protect 23. But Cornyn said he has had to commit cash and staff to places that were supposed to be safe, like North Dakota, Indiana and Arizona.

“The map has shifted a lot,” he said.

There’s also a personal stake in this for Cornyn, whose bid to become the Republican whip, a powerful Senate leadership position and political stepping stone, could hang in the balance. But the Texan, first elected to the Senate in 2002, said he is putting that off until after the November elections.

Cornyn has had a hectic travel and fundraising schedule as campaign chief. Texas and New York have been recent fundraising stops in a campaign that has raised nearly $100 million.

But his biggest challenges have been dealing with the unforeseen: The primary defeat of a Republican elder statesman, Sen. Richard Lugar of Indiana, to a tea party candidate; the surprise retirement of Sen. Olympia Snowe of Maine, which put her state in play for the Democrats; and the unexpected primary win in Missouri of Rep. Todd Akin.

It is Akin who has given Cornyn the most heartburn, with his remarks about how victims of “legitimate rape” have ways to stop from becoming pregnant. Cornyn joined many Republicans in repudiating the conservative suburban St. Louis congressman and yanked funding from his race in an attempt to force him drop his candidacy, which he didn’t.

The subject remains a clear frustration for Cornyn.

“I don’t want to talk anymore about Missouri,” Cornyn said. “We still hope that Claire McCaskill is defeated. We’d much prefer Todd Akin.”

But with Akin as her opponent, McCaskill, the Democratic incumbent, has seen her chances for re-election increase. She recently unleashed a series of sharply critical ads about his rape comments.

“I’ve just taken Missouri off the map” as competitive, said Jennifer Duffy, Senate analyst at the non-partisan Cook Political Report. “Cornyn needs to worry about more than Missouri. They had to protect the rest of their races.”

Indeed, political scientist Norm Orenstein of the American Enterprise Institute suggested that the tide appears to be shifting.

“Missouri was as close to a slam-dunk as they had,” he said. “Odds are 60 percent that Democrats retain the majority.”