The Republican presidential field has a new rising star in its crosshairs

Sen. Marco Rubio's (R-Florida) rise in Republican primary polls has led to a steady increase in attacks from his presidential rivals.

Some of those attacks have been less subtle than others.

Real-estate mogul Donald Trump has been among Rubio's loudest critics, unloading shot after shot at the senator in a single interview this week, even going as far as to mock pundits for praising his good looks.

"I think I'm better looking than he is," Trump mused in a Monday interview with Bloomberg Politics.

On Tuesday, Trump continued to fire away at Rubio, calling his personal finances a "disaster" and saying he lives above his means. Among other things, Trump has also called Rubio sweaty, overrated, too dependent on water, weak on illegal immigration, and a "kid."

But Trump is hardly the only Rubio antagonist these days. Notably, former Florida Gov. Jeb Bush (R) directly confronted Rubio, his former ally in the Tallahassee statehouse, at last week's CNBC debate. Bush blasted Rubio for his poor US Senate-attendance record and told him to either show up to work or resign.

Bush's campaign has further compared Rubio unfavorably to President Barack Obama, who was also a first-term US senator before he ran for president. And US News & World Report's David Catanese revealed last week that the Bush campaign had presented some harsh anti-Rubio opposition research in an internal strategy briefing.

Two other presidential candidates who generally resist criticizing fellow Republicans have also found ways to sharply contrast their records with Rubio's.

"It's hard to keep up with how many times Marco changes his positions on these things, to tell you the truth," New Jersey Gov. Chris Christie (R) said of Rubio's immigration positions in a Tuesday interview with conservative radio host Laura Ingraham that was flagged by The Weekly Standard.

Sen. Ted Cruz (R-Texas), meanwhile, went out of his way to note he and Rubio have some policy disagreements during a Monday interview with conservative radio host Hugh Hewitt.

"I believe in free trade, but I voted against TPA, and that's an area where there's a difference," Cruz said, using the acronym for trade promotion authority while discussing a potential landmark Pacific Rim trade agreement. "So for example, Marco Rubio, who's in the Senate, he voted in favor of TPA. I voted against it. So there's a clear difference in terms of records among the candidates on this."

These shots followed Rubio's slow-but-steady rise in the polls, as he has eclipsed Bush and other contenders to become the No. 3 Republican in the RealClearPolitics average of national surveys. Trump and retired neurosurgeon Ben Carson still have a healthy lead in the crowded field.

For his part, Rubio generally shrugs off the attacks. When Bush confronted him at the debate, Rubio deftly told Bush that "someone has convinced you that attacking me is going to help you," a line he repeats whenever he is asked about his former ally.

However, Rubio did return some fire at Trump during a Wednesday interview on Fox News. Rubio, who is frequently accused of flip-flopping on his support for comprehensive immigration reform, declared that Trump was the true flip-flopper on the issue.

"Donald was a supporter of amnesty and of the Dream Act," Rubio said. "And he changed his positions on those issues just to run for president."

Rubio soon added: "We're going to continue to do what we're doing. That's all I can control, is my campaign and what we talk about. And we're going to continue to talk about our vision for America's future and our plan to get us there. And I have confidence that voters will choose us in the end."