Just last week, former Vice President Mike Pence said he hoped federal prosecutors would not bring charges against former President Donald Trump. On Wednesday, a day after Trump was arraigned on dozens of felony counts related to classified documents, Pence described the allegations as “a very serious matter." “I cannot defend what is alleged,” Pence, who is now challenging Trump for the Republican presidential nomination, said on CNBC. The former vice president's evolving message highlights the high-stakes dilemma for Trump's Republican rivals, who are struggling to find a clear and consistent strategy to take on the frontrunner as Trump’s unprecedented legal troubles threaten to dominate all other issues in the 2024 presidential contest. Some Republican leaders this week have demonstrated a newfound willingness to criticize Trump over the seriousness of the allegations, which include mishandling government secrets that as commander in chief he was entrusted to protect. Florida Gov. Ron DeSantis, a former naval officer and Trump's top rival for the nomination, said that “if I would have taken classified (documents) to my apartment, I would have been court-martialed in a New York minute.” But that was just a brief mention in a weekend speech at a North Carolina GOP gathering, during which he focused his censure on the Justice Department and the Biden administration. It's been much the same for other challengers. Even the most aggressive have layered their criticism of Trump with attacks against the Justice Department — for bringing charges against him — that make it difficult at times to determine exactly where they stand on the former president. And that’s precisely the point, given Trump’s continued popularity among GOP voters and his rivals' desire to dent his lead without alienating his base. Indeed, most of Trump's competitors are making a risky bet — for now — that the weight of his extraordinary baggage will eventually sink his reelection bid. They believe it will take time. Trump's Republican opponents privately concede that the former president’s considerable political strength is likely to grow stronger, at least in the short term, as GOP voters, key officials and conservative media leaders rally around him. For example, Pastor Robert Jeffress, of the First Baptist megachurch in Dallas, initially declined to endorse Trump's 2024 bid but declared Tuesday night that the GOP's presidential primary was all but over. “I thought there would be almost a civil war in the Republican Party for the nomination, but that quickly turned into an unconditional surrender,” said Jeffress, who mingled at Trump's post-indictment gathering at Bedminster, New Jersey. "People absolutely love this president, and I believe his base is going to turn out.” The Republican establishment has tried and failed to reject Trump and his divisive politics for much of the last decade. But this time the GOP faces the very real possibility that a man who has been indicted twice and charged with dozens of felonies could become the party’s standard-bearer in 2024. Fighting that outcome, which once seemed all but inevitable, a powerful conservative voice is being raised in the fight for the first time. The Koch network’s political arm, Americans for Prosperity, has begun running online ads across Iowa, New Hampshire and South Carolina — the first three states on the GOP’s presidential primary calendar — focusing on questions about Trump’s electability in next fall’s general election against Biden. The new ads make no mention of his legal troubles. “Trump did a lot of good things as president," one of the ads says. “But this time, he can’t win.” Americans for Prosperity CEO Emily Seidel said her organization has talked to thousands of voters in key states to determine the most effective arguments to undermine Trump’s political strength. “Based on the data we’re collecting, more than two-thirds of people who say they’re supporting Trump are also receptive to arguments that he is a weak candidate, his focus on 2020 is a liability, and his lack of appeal with independent voters is a problem,” Seidel said. "That tells us that many Republicans are ready to move on — they just need to see another candidate step up and show they can lead and win.” So far, Trump's rivals are still trying to find their footing as the former president commands a big lead in early Republican primary polls. And as they test evolving messages on the campaign trail and in media appearances, none of top-tier competitors are running paid advertisements seizing on Trump's legal troubles. Republican presidential contender Nikki Haley, the former ambassador to the United Nations in the Trump administration, told Fox News on Monday that Trump was incredibly “reckless with our national security” if the allegations in the indictment are true. On Tuesday, she repeated the pointed criticism, but also said she’d be inclined to pardon Trump if he’s convicted. “I think it would be terrible for the country to have a former president in prison for years because of a documents case,” Haley said on the Clay Travis and Buck Sexton radio show. Others have made defending Trump a central message in their early campaigns. Speaking outside the Miami courthouse on Tuesday, White House hopeful Vivek Ramaswamy announced that his campaign had sent a letter to other 2024 candidates challenging them to join his pledge to pardon Trump on their first day in office. “I respectfully request that you join me in this commitment or else publicly explain why you will not," it read. Trump, meanwhile, is trying to take advantage of the media storm. After his appearance in federal court in Miami, he made a stop at the city's famed Versailles Restaurant in Little Havana, with news cameras in tow. He then headed home to his Bedminster summer residence, where aides had assembled hundreds of supporters, club members and reporters for a post-arraignment speech. Trump was welcomed like a general returning home from battle. Insisting he was innocent of all charges, the former president vowed that, as president, he would appoint a special prosecutor to investigate Biden and his family. As for the indictment and charges? “This is called election interference and yet another attempt to rig and steal a presidential election,” Trump said. As they reckon with the fact that Trump faces years behind bars, as well as the logistical complications of balancing court appearances with campaign rallies, Trump's political advisers have stressed what they see as the political benefits. They believe the wall-to-wall coverage of his legal woes makes it difficult for his competitors to be heard, a point that other campaigns acknowledged privately. “From a campaign standpoint, I mean, what did the other candidates do today? Do we know?” asked Trump campaign spokesperson Steven Cheung. “There’s no oxygen for the other candidates." Those other candidates are eager to highlight cracks in Trump's support, although for now, they appear to be modest. On Capitol Hill, a small but growing Republican minority of lawmakers have recently described the new federal charges against Trump as serious. “I would not feel comfortable with a convicted felon in the White House,” Rep. Ken Buck, R-Colo., told CNN. He pointed to Trump’s attacks against Hillary Clinton, who was accused of mishandling classified documents in her emails in 2016. “His words have set the standard.” Veteran Republican strategist Ari Fleischer warned that it would take time to understand the political impact of Trump's growing legal challenges. “A short-term rally around Trump now is not the true measure," Fleischer said. “The only test is a long-term test.” ___ Peoples reported from New York. Associated Press writer Ali Swenson in New York contributed to this report. Read More Ivanka and Jared split over attending Trump 2024 launch – follow live Why was Donald Trump impeached twice during his first term? 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