Republican Texas Sen. Ted Cruz warned Republicans against underestimating Vice President Kamala Harris as she emerges as the top contender for the Democratic Party's nomination for president.

"Republicans, I worry, vastly underestimate Kamala Harris. They don't think very highly of her. They don't think she's terribly bright. When you or I bring up Kamala Harris' name in Republican circles, people laugh. It's immediately a punchline," Cruz said Monday on his podcast "Verdict with Ted Cruz." 

Cruz warned against Republicans preemptively celebrating a Trump-Vance win months out from the election, arguing Democrats and the media will promote Harris as an "historic" candidate. 

"I think people are underestimating what billions of dollars of free media, of the entire corrupt corporate media complex, pitching her as a combination of Mother Teresa, Oprah and Gandhi," Cruz said. 

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"I still think Trump wins in November, but this is not a layup. It is not given.… If you're a Democrat, what makes you nervous is chaos, and this much chaos 100 days out is scary. But you know what? Even more scary is going to an election where you're almost certain to lose, which is where Biden was," he continued. 

President Biden announced Sunday afternoon on X that he was bowing out of the presidential race, which was quickly followed by him endorsing his vice president to run in his place. 

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"My very first decision as the party nominee in 2020 was to pick Kamala Harris as my Vice President. And it’s been the best decision I’ve made. Today I want to offer my full support and endorsement for Kamala to be the nominee of our party this year. Democrats — it’s time to come together and beat Trump. Let’s do this," Biden said in a follow-up social media post endorsing Harris. 

The vice president held her first public address Monday since Biden's announcement, praising Biden for his decades in government. 

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"I am a firsthand witness that every day our President Joe Biden fights for the American people and we are deeply, deeply grateful for his service to our nation," she said Monday from the South Lawn of the White House during an event celebrating NCAA athletes. 

Harris is not yet the official nominee of her party, as the DNC must first certify her – or another potential candidate – next month. 

Former President Trump officially became the nominee of the Republican Party last week after announcing Ohio. Sen. JD Vance as his running mate. Cruz said the RNC, which was held in Milwaukee, Wisconsin, was the "best" he's attended in all his years in politics. However, he remarked it was concerning to see some in the GOP were "overconfident" that Trump has an election win locked. 

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"I was very worried at the convention," Cruz said. "… I am afraid people were overconfident at the convention. There was an air of celebration. It was, 'We've won. We're on to victory. This is a landslide. Trump's coming back in. We've got a huge Republican majority.'"

"In my view, and I was trying to say this, 'Look, there's a time for celebration. Celebrate after Election Day. Celebrate after we've won. Now is not the time for celebration. Now is the time for hard work.'"

Get the latest updates from the 2024 campaign trail, exclusive interviews and more at our Fox News Digital election hub.

Secret Service Director Kimberly Cheatle opened her testimony to Congress with a frank admission that her agency "failed" in its mission to protect former President Trump.

Cheatle testified before the House Oversight Committee on Monday, facing a grilling from Republican and Democratic lawmakers alike. Several GOP lawmakers have called on her to resign, though she has so far refused.

"The Secret Service's solemn mission is to protect our nation's leaders. On July 13, we failed," Cheatle said in her opening statement. "As director of the United States Secret Service, I take full responsibility for any security lapse of our agency. We are fully cooperating with ongoing investigations. We must learn what happened, and I will move heaven and earth to ensure that an incident like July 13th does not happen again."

Despite saying that she takes full responsibility for what happened, Cheatle has said she will not resign, stating during the hearing that she believes she is currently the best person to lead the Secret Service right now.

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Cheatle made clear that none of her testimony should be understood as a criticism of local law enforcement or other security partners the Secret Service worked with at former President Trump's rally in Butler, Pennsylvania.

Chairman James Comer, R-Ky., opened his questioning by asking whether a Secret Service agent had been on the roof the shooter used at any point on the day of the rally.

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Cheatle skirted the question, however, saying she could not offer specific details due to ongoing investigations. Comer then pressed about whether the building was within the security perimeter around Trump's rally, to which Cheatle said it was not.

Cheatle later confirmed to Rep. Jamie Raskin, D-Md., ranking member of the committee, that shooter Thomas Matthew Crooks was identified as a suspicious individual before Trump was taken onto the stage. He pressed Cheatle on why Trump was allowed on the stage despite the risk, and the official stated that while Crooks had been identified as suspicious, he had not yet been determined to be a threat.

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Cheatle will spend several hours fending off questions from lawmakers throughout Monday's hearing. Comer set the tone for the day with his own opening remarks, in which he called on the director to resign.

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"It is my firm belief, Director Cheatle, that you should resign. However, in complete defiance, Director Cheatle has maintained she will not tender her resignation," Comer said in his remarks. "Therefore, she will answer questions today from members of this committee seeking to provide clarity to the American people about how these events were allowed to transpire."