Senator Marco Rubio. (Andrew Harrer/Bloomberg)

Ahead of Thursday night's Republican presidential debate in Houston, Sen. Marco Rubio (R-Fla.), who has long treaded carefully around Donald Trump, is signaling a more aggressive posture toward the GOP front-runner.

In remarks in Houston on Wednesday, Rubio took the rare step of attacking Trump by name at a campaign rally. And on Thursday morning, his campaign distributed to reporters a New York Times story about Trump's Mar-a-Lago Club in Palm Beach, Fla., filling hundreds of positions with guest workers, even as he has espoused hard-line views on immigration.

While it's not clear whether Rubio will continue his assault during Thursday's debate, the moves mark a notable escalation in Rubio's criticism of Trump, which until now has been generally been less direct. It also comes at a time when Trump threatens to run away from Rubio and the rest of the field. A new poll out Thursday shows Trump leading Rubio by 16 points in a his home state of Florida.

Asked about the signs of a shift, Rubio spokesman Alex Conant declined to discuss the campaign's strategy.

Speaking in Houston on Wednesday, Rubio, a staunch defender of Israel, said "the front-runner in this race, Donald Trump, has said he's not going to take sides on Israel versus the Palestinians because he wants to be an honest broker," the Associated Press reported. Rubio also accused Trump of believing "parts of Obamacare are pretty good."

Rubio usually takes indirect shots at Trump during campaign events. For example, he has said routinely on the campaign trail that "anger is not a plan." In two different states  Tuesday, Rubio said the election cannot be about "making a point."

Asked on his campaign plane Tuesday why he does not tend to mention Trump's name to his crowds, Rubio responded: "That's how I speak and that's how my campaign's going continue to be."

When he fields questions from reporters about Trump, Rubio is more direct. He said on his plane that "65 to 70 percent" of Republican voters do not want to support Trump.

Rubio's strategists have long felt that hitting Trump hard before the race has narrows dramatically is perilous. Trump's counter-punches have been devastating, and they always attract widespread attention. So, the thinking goes, holding their fire until the field pares protects them against becoming a Trump target.

But time is not on Rubio's side. None of the other three candidates are signaling that they are preparing to drop out. Meanwhile, Trump is on a three-state winning streak with no signs off slowing down. Rubio's backers worry that if he does not defeat Trump in his home state of Florida on March 15, his campaign will effectively be finished.

At Rubio campaign rallies, supporters have started expressing their unhappiness with Trump more overtly. At a rally in Michigan on Tuesday, one person held a up sign encouraging the country to build bridges not walls, a reference to Trump's oft-stated call to build a wall on the U.S.-Mexico border and have Mexico pay for it. Another called for Trump's "defeat."

Earlier this week in Nevada, Trump foreshadowed that he will come back forcefully if Rubio decides to attack him.

"I've been very nice to Rubio, because he hasn't hit me," he said. "When he does, you will see what happens."