LAS VEGAS — Ted Cruz fired a top campaign aide Monday after discovering he distributed a doctored video that made it appear as if Sen. Marco Rubio was bad-mouthing the Bible.

The video in question, promoted on social media by communications director Rick Tyler, showed Rubio walking past Cruz's father and a staffer who was reading the Bible, and includes subtitles indicating that Rubio mocked the Bible as he walked past. The captions made it appear as if Rubio was saying there are "not many answers" in the Bible.

Tyler eventually removed the video from his social media platforms and posted a lengthy apology on Facebook for promoting it. But Cruz told reporters during a news conference here on Monday just before joining a campaign rally with supporters at a local YMCA that he wouldn't tolerate personal attacks from his staff directed at opponents, even if by mistake.

"Yesterday a staffer from our campaign sent out a tweet, that tweeted a news story, that purported to indicate Marco saying something negative about the Bible," Cruz said. "The news story was false; that staffer deleted the tweet, apologized and pulled it down — although I've spent this morning investigating what happened. And, this morning, I asked for Rick Tyler's resignation. I have made clear in this campaign that we will conduct this campaign with the very highest standards of integrity."

Cruz also said there was no truth to the rumors that he and Rubio met privately on Monday in Las Vegas, nor had the two spoken. The Rubio campaign did not respond to emails requesting comment on Tyler's firing. Cruz campaign spokeswoman Catherine Frazier declined to elaborate on how the dismissal was handled, saying only that she was going to let the senator's comments speak for themselves.

Tyler, Cruz's communications director, has been with the Texas senator from the beginning of his campaign and played a key role in crafting the candidate's message over the past 11 months.

In the wake of Cruz's disappointing third place finish in South Carolina, this move by Cruz to fire Tyler suggests that attacks by his opponents that he was running a "nasty" campaign that played fast and loose with the truth were having an impact that needed to be addressed.

Cruz's liability in the presidential contest has been low likability and the reputation for being a firebrand. These personality challenges haven't hurt Cruz with his core supporters, but they do threaten to blunt his path to the nomination by limiting his ability to grow his support. Despite the fact that Cruz won the Iowa caucuses, Donald Trump is poised to run away with the nomination and Rubio could surge.

"Rick is a really good spokesman who had the unenviable task of working for a candidate willing to do or say anything to get elected," said Rubio communications director Alex Conant. "There is a culture in the Cruz campaign, from top to bottom, that no lie is too big and no trick too dirty. Rick did the right thing by apologizing to Marco. It's high time for Ted Cruz to do the right thing and stop the lies."

The news about Tyler and what led Cruz to fire him isn't necessarily just the pre-occupation of political reporters. Shawn Preston, 49, a Las Vegas voter who likes Cruz and plans to caucus for him on Tuesday, heard news of Tyler being let go, and said it was the right thing to do.

Preston, who attended the Monday afternoon rally at the YMCA, said he didn't agree that Cruz was running an unacceptably negative or truth-challenged campaign. But he was already aware of the story behind Tyler's resignation and that Cruz had dismissed him when asked about it by the Washington Examiner, even though the senator made the announcement only minutes before he took the stage here to rally supporters.

"I saw it and he actually fired his guy today — but he ended up firing him for a negative campaign against Rubio," Preston said. "It was justified. He needed to do that."

Trump has relentlessly called Cruz a "liar," and Rubio has accused the senator of running a campaign that goes negative beyond the acceptable standards for a hard-fought presidential campaign.

In a memo the Rubio campaign issued recapping the South Carolina primary, where the Florida senator finished second, less than 1 percentage point ahead of Cruz, it charged that the senator "will truly say anything at all to get elected and ran the nastiest campaign South Carolina has ever seen," and that his "dishonest tactics have left him with permanent damage."

Cruz, as he has for days, pushed back. Even as he announced Tyler's departure to reporters, the senator said Trump and Rubio are lying about him because they can't defend their records and positions.

"At the last debate in South Carolina, I pointed out Donald Trump's passionate support for Planned Parenthood, he responded by bellowing, 'liar,' " Cruz said. "Both Donald Trump and Marco Rubio's campaign have also relied on fabrications — utter fabrications."

Tyler is a veteran Republican operative who four years ago advised a super PAC supporting presidential candidate Newt Gingrich, the former House speaker.