Marco Rubio is gaining in a key metric that reliably predicts who wins presidential nominations

Sen. Marco Rubio's (R-Florida)surge in polls has gone hand in hand with increasing strength in a key metric that serves as one of the best predictors of electoral success.

On Wednesday, Sen. Jim Risch (R-Idaho) announced that he is supporting Rubio for president. Risch is the third senator to come out in favor of Rubio in three days, following the endorsements of Sen. Steve Daines (R-Montana) on Tuesday and Sen. Cory Gardner (R-Colorado) on Monday.

Many electoral statisticians have found that endorsements from national political elites are often a much stronger predictor of who will eventually win the nomination. Researchers have found that since 1980, the "single best predictor" of an eventual nominee comes in the number of endorsements from party elites. 

Former Florida Gov. Jeb Bush (R) still has the most endorsements of any Republican candidate — he's garnered the support of several senior US senators like Sens. Orrin Hatch (R-Utah) and Susan Collins (R-Maine), along with almost two-dozen members of the House of Representatives.

But as FiveThirtyEight's Nate Silver noted, Rubio has rolled out the most major endorsements over the last two months — by far. While Bush has only announced two endorsements in the last month from members of Congress, Rubio has rolled out seven, more than every other Republican candidate combined.

Other Republican candidates have clearly begun to notice Rubio's rising status after his strong performance in last week's debate.

Real-estate magnate Donald Trump has increased his attacks on Rubio, criticizing everything from the senator's immigration policy stances to his looks.

"I think he's a highly overrated person. I've called him a lightweight. I think he's a lightweight. I hope I'm wrong about that," Trump said in a Monday interview with Bloomberg Politics.

And Bush has gone after Rubio's attendance record in the Senate. Bush's campaign also featured attacks on Rubio in a presentation to donors last week.

"We need to offer a contrast to the current president. Hillary will pitch competence and experience. Marco is a GOP Obama," the presentation said. "Rubio and President Obama have strikingly similar profiles: first-term senators, lawyers and university lecturers, served in part-time state legislatures for eight years, had few legislative accomplishments, and haven't shown much interest int he process of advancing legislation and getting results."

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