JOHNSTON, Iowa (AP) — In a swipe at the his party’s front-runners, Republican White House hopeful Marco Rubio charged Saturday that anger alone isn’t enough to qualify someone to be president.

“Being angry about the direction of our country by itself will not be enough,” the Florida senator said, as he courted Iowa voters ahead of the state’s Feb. 1 caucuses. “We also have to have someone for president who knows exactly what they’re going to do when they get there.”

Rubio called out leading candidates, real-estate mogul Donald Trump and Texas Sen. Ted Cruz, by name during a town hall-style meeting in Johnston, Iowa, the first of four scheduled campaign appearances Saturday. Trump and Cruz are surging in the polls, in part, by successfully tapping into conservative anger about the direction of the country.

Republican presidential candidate Sen. Marco Rubio (R-FL) participates in the Fox Business Network Republican presidential debate at the North Charleston Coliseum and Performing Arts Center on January 14, 2016 in North Charleston, South Carolina. The sixth Republican debate is held in two parts, one main debate for the top seven candidates, and another for three other candidates lower in the current polls. (Photo by Scott Olson/Getty Images)

Republican presidential candidate Sen. Marco Rubio (R-FL) participates in the Fox Business Network Republican presidential debate at the North Charleston Coliseum and Performing Arts Center on January 14, 2016 in North Charleston, South Carolina. (Photo by Scott Olson/Getty Images)

In a debate this week, Trump declared that he “gladly” accepted “the mantle of anger.”

“Obviously, I share the frustration that others have about the future of our country and where it’s headed. But I’m going to detail exactly what we’re going to do about it,” Rubio said. “Donald hasn’t done that, whether it’s on national security or on the other issues. I think that’s a huge difference.”

Rubio also condemned Senate colleague Cruz for supporting federal budgets that would decrease defense spending and for proposing a “European-style” value-added tax. Cruz’s tax plan, in particular, would “really hurt seniors,” Rubio said.

And repeating criticism he outlined in Thursday’s debate, Rubio likened New Jersey Gov. Chris Christie’s policies to those of President Barack Obama. The Florida senator accused Christie of supporting Common Core education standards, gun control and the Supreme Court nomination of Sonia Sotomayor.

“I would never support someone like that to the Supreme Court,” Rubio charged.

Christie has disputed the assertion that he backed Sotomayor, but in 2009 Christie said, “I support her appointment to the Supreme Court and urge the Senate to keep politics out of the process and confirm her nomination.”

Rubio promised to avoid personal attacks in the days leading up to the Iowa caucuses, but would continue to outline policy differences between his Republican rivals.

“These are policy differences,” he said. “We’re all going to be on the same team soon enough.”

Rubio has been criticized for not spending as much time on the ground in early voting states as some of his Republican competitors. He chartered a private plane to make four stops on Saturday and another five on Monday.