Marco Rubio has been downplaying his rise in the polls, aiming to avoid peaking too soon and instead, evolving as the the survivor of a long, drawn-out Republican primary.

But someone has taken notice.

Donald Trump is now on to Rubio, the Florida senator currently in fifth place in the RealClearPolitics average, within a few tenths of a percent of Jeb Bush. Only Trump, Ben Carson, and Carly Fiorina are ahead. During a campaign stop in South Carolina and in subsequent media interviews, Trump made Rubio his new target, criticizing him for being “sweaty,” “overly ambitious,” financially unsuccessful, absent from the U.S. Senate, and “a kid.”

Rubio has tried to stay out of Trump’s way, opting instead to focus on fundraising and campaign activity and hoping to stay under the radar while his rivals duke it out. Now that he is on Trump’s hit list, Rubio figures he can’t just stand idly by any longer.

On Thursday, during an interview with Kentucky Sports Radio, Rubio called Trump “a touchy and insecure guy.”

“He takes shots at everybody that gets anywhere close to him, in terms of a poll, or anytime he hits a rough spot, that’s what he does,” Rubio said, noting Trump had a poor debate performance last week.

Rubio suggested that Trump’s “10-second sound bite” campaign is wearing thin. “I think he’s kind of been exposed a little bit over the last seven days,” the senator said, before turning back to his own campaign.

While Rubio has been reluctant to go after Trump—he has argued that if he perpetually responds to the current frontrunner, his campaign would ultimately become consumed by it—a lot has changed in the GOP field in the past seven days. And those changes put an unwanted spotlight on Rubio.

This week, Wisconsin Gov. Scott Walker suspended his falling and financially troubled campaign. His exit leaves more room for candidates like Rubio, operating in the more establishment-oriented, non-Trump lane. Several Walker supporters jumped over to Rubio’s side this week. Walker was originally seen as the top challenger to Rubio in overcoming his better-financed and well-staffed rival, Jeb Bush. With Walker gone, Rubio is a step closer.

Rubio also had a well-received debate performance last week, besting his rivals (aside from Carly Fiorina) onstage. His poll numbers have since risen nationally. In addition, a Florida Atlantic University poll found him leading Bush in the Sunshine State.

But while many candidates try to capitalize on new positive polls, Rubio downplays them. “These polls don't really mean anything at this stage,” he told ABC’s “This Week.” “I'm not sure the mid-September winners are where you want to be."

The freshman senator has been especially conscious of rising too soon. The perils of early frontrunners hit home this week with the exit of Walker, who was doomed by an early lead that revealed him to be unprepared for prime time. Walker's sizable campaign couldn't be supported when fundraising began to dry up.

Rubio has been trying to run a budget campaign, with a small staff and headquarters. While he is competing in all of the early states—unlike some rivals, who are concentrating on either Iowa or New Hampshire—Rubio has also been criticized for being absent on some parts of the trail. Bloomberg reported this week that some Granite State activists worry he is missing opportunities to gain ground there.

The scenario has become a familiar one for the Rubio campaign. He has had breakout moments either in the debates or on the campaign trail, but he downplays them and goes back under the radar. It’s all part of a strategy of not surging too soon. In some ways, Rubio is hoping to be the accidental candidate, the one left standing after the rivals flame out.

“We need everybody not named Marco to fizzle. That is the plan,” said campaign manager Terry Sullivan at a National Review event last week. “We need everybody to slowly fizzle out, and we think they will.”

There are perils to this strategy, of course, including getting lost in the shuffle or appearing disengaged. And Rubio’s path in terms of which primary states he can win is not yet clear.

But in a Republican primary where conventional wisdom and traditional campaigning have been overtaken by outsiders and experimentation, this slow, steady, shoestring campaign could prove successful.

Still, as Rubio found this week, one can stay under the radar only so long. Especially when Trump comes calling.