Rubio, Cruz emerge as deftest debaters, but can they make it last?

Four Republican debates have shown the GOP's deftest debaters are two senators, Marco Rubio and Ted Cruz. The coming months will show if that helps them overcome the two front-runners, Donald Trump and Dr. Ben Carson.

For the second consecutive encounter, the two 44-year-old freshman senators gave strong performances, this time in the Fox Business Network-Wall Street Journal debate focused on economic issues. Rubio sidestepped potential land mines, spoke strongly on foreign policy and touted his general election credentials, while Cruz reeled off the crisp answers aimed at solidifying his credentials as the most consistently conservative GOP candidate.

Recent polls showed the two senators gained support after last month's Colorado debate but still trail Trump and Carson, both nationally and in next year's first three battlegrounds: Iowa, New Hampshire and South Carolina. Without a drop in support for either the billionaire businessman or the retired neurosurgeon, who together hold more than half of current GOP support, Rubio and Cruz might find it hard to catch up by the Feb. 1 Iowa caucuses.

Tuesday night illustrated their different strategies. By stressing conservative stances, Cruz underscored his hope of cutting into Carson's evangelical support in Iowa and South Carolina. Rubio hopes to gain backing within the establishment wing, in part by portraying himself as the ideal GOP rival for Democratic front-runner Hillary Clinton.

Trump's signature proposal to deport illegal immigrants came under fire from both Ohio Gov. John Kasich and former Florida Gov. Jeb Bush, and he seemed surprisingly dovish in saying, "We can't continue to be the policeman of the world." Carson, who responded to a question about challenges to his resume with an uncharacteristically biting rejoinder accusing Clinton of lying about Benghazi, was ignored by his rivals but struggled with substantive questions on taxes and Syria.

Trump's plan to deport 11 million illegal immigrants sparked the first sharp exchange. "We all know you can't pick them up and ship them across, back across the border," Kasich said. "It's a silly argument. It makes no sense." Bush agreed. "It's not embracing American values. And it would tear communities apart," he said, adding, "They're doing high-fives in the Clinton campaign right now when they hear this." Cruz backed Trump, saying, "If Republicans join Democrats as the party of amnesty, we lose." But Rubio, who has hardened his once-moderate immigration stance, stayed out of the crossfire.

Later, however, the Florida senator was asked whether his proposal for a new $2,500 child credit was "another expensive entitlement." He defended it as "pro-family," but Kentucky Sen. Rand Paul challenged him.

"Is it fiscally conservative to have a trillion-dollar expenditure?" he asked. "Add that to Marco's plan for a trillion dollars in new military spending, and you get something that looks to me not very conservative." "I know Rand is a committed isolationist," Rubio shot back. "I believe that the world is a stronger and a better place when the United States is the strongest military power in the world." Later, Fox Business Network anchor Maria Bartiromo lobbed Rubio a softball, asking why Americans should trust him over Clinton's greater experience. "The election is actually a generational choice," Rubio replied, echoing his campaign's theme. "This election is about the future." Cruz, who relentlessly attacked Washington as "fundamentally corrupt," had a specific answer when asked about offsetting his tax cut proposals, citing $500 billion in spending cuts, including eliminating five agencies. But in a flub reminiscent of former Texas Gov. Rick Perry's four years ago, the Texas senator named only four, citing the Department of Commerce twice and omitting the Department of Education. However, no one caught it on a night moderators were relatively restrained in pressing candidates.

Many Bush answers were knowledgeable but nonconfrontational. But former corporate executive Carly Fiorina inserted herself so strongly with characteristically direct, sharp responses that Trump asked, "Why does she keep interrupting everybody?" But she has failed to translate prior strong debate challenges to Trump into lasting gains, showing debates sometimes have limited impact.

The post-debate challenge for Rubio and Cruz will be to avoid that kind of drop-off and hope Trump or Carson falter enough to help them make the showing in Iowa, New Hampshire or South Carolina that propels them into the top tier by the delegate-rich primaries in March.

Carl P. Leubsdorf is the former Washington bureau chief of the Dallas Morning News.