"You can't predict a campaign," said Roberts, an ex-Marine who vowed to battle hard for his seat during the homestretch. "Fight's a good way to put it.
"Yeah, this is a close race, but I'm going to win," he said. "And I'm going to win it because people know that the road to a Republican majority runs right through Kansas. And a vote for me will be a vote for that Republican majority that will put the brakes on President Obama."
Read MoreThis heartland battle may tip control of the Senate
He ascribed Orman's success to far to millions of dollars of TV ads, aired before he started running his own. He also acknowledged that Orman has made widespread gains on unhappiness with both parties.
"I think that my opponent has tapped into what is very real public dissatisfaction with not getting things done in Washington," he said. "Everybody has some blame in that."
He didn't apologize for campaigning alongside Sen. Ted Cruz of Texas, a hero among conservative voters he needs but a symbol of partisan warfare after last fall's government shutdown.
"I don't think he promoted the government shutdown," Roberts insisted. "Everybody says that. That's not the case. What he did is, he took a whole filibuster and discussed every facet of Obamacare. He's the first one to really alert the American public, and for that matter senators, and said 'This is not going to work. This will be a disaster.' And he was right, and I credit him for that."
Read MoreBerkshire's Buffett predicts Hillary Clinton will win 2016 presidency
Because Kansas is such a conservative state—it has voted Republican for president in 10 straight elections—Roberts' principal message now is that Orman is hiding traditional Democratic beliefs behind the cloak of independence.
"Voters now in Kansas are finding out who he is," Roberts said, standing outside his campaign bus at the outset of a tour across the state. "He's a liberal Democrat. He ran against me in 2008, he donated all these dollars to Obama, to Hillary Clinton, to Harry Reid. By the stand he takes on the issues: Obamacare, he's for that. He's against the pipeline. First and Second amendments, there are changes he'd like to make. He is not an independent."
And the ex-Marine had a message for any Republican preparing to blame him for another GOP setback this fall: he'll overtake Orman by Nov. 4.
"I've always fought every day or I wouldn't ever enter any race," Roberts said emphatically. "Semper Fi —Marines take the hill."