U.S. Sen. John Cornyn, R-Texas, spoke to students of KIPP San Antonio's University Prep high school during a short visit Friday and told them the most difficult part of his job is his schedule. 

Trying to organize fellow Republicans in Texas might be a bit tougher. Fellow Texas Republican Sen. Ted Cruz has not publicly endorsed him for re-election.

"This is all more or less to be expected and it's manageable, but I hope after the primary is over we can all come together and be unified, because then we're much more likely to be successful," said Cornyn. "The Republican coalition is composed of libertarians, social conservatives, Tea Party, fiscal conservatives, national security hawks and we need as many people to come within that umbrella to vote for our candidates and to vote in order to be able to govern."

Nearly every faction seems represented on the ballot, with seven other challengers -- only one of whom Cornyn has met. 

Chris Mapp, a candidate from Port O'Connor, has spoken out before against Cornyn for not siding with Cruz during the battle against the Affordable Care Act.

"Our current U.S. senator is not bringing unity to our party," said Mapp. "We can no longer as conservatives accept the status quo establishment politician. It's time for a fresh voice."

Retired Army Lt. Col. Ken Cope discussed Cornyn's immigration record as a need for change.

"Twelve years in the United States Senate on the immigration committee and he has watched immigration go from a social problem to a national defense issue on the border. And he's done nothing," said Cope. "He's been basically just warming the seat and not standing up and fighting for Texans."

A recent poll indicated that less than 50 percent of registered Republican voters would select Cornyn in the primary, causing some speculation of a May runoff. 

"I've been around the block enough to know that not all polls are created equal," said Cornyn. "I feel pretty confident about going into the primary, but I'm not going to take it for granted."

Early voting runs through Feb. 28. The primary election is March 4.