AP Photo/John Locher
Republican presidential candidate Sen. Marco Rubio, R-Fla., speaks with the media after touring the World Famous Gold & Silver Pawn shop with owner Rick Harrison Thursday, May 28, 2015, in Las Vegas.
The CNN poll showed that Rubio's support has plunged to 6% among likely Republican primary voters. In May, he was leading the GOP pack with 14%.
This drop represents the largest shift among any Republican candidate. While most candidates remained statistically within the margin of error, Rubio's drop is the only significant movement up or down among candidates. He also lost ground in a theoretical head-to-head matchup with Democratic frontrunner Hillary Clinton - he now trails her by 17 points after coming within three in May.
Wisconsin Gov. Scott Walker (R), who has yet to officially enter the race, also dropped several percentage points to 6%, but he was within the poll's 5% margin of error.
Several factors may be behind Rubio's drop. For one, Rubio didn't grab many headlines during the week that the poll was taken like some of his opponents - including real-estate mogul Donald Trump, who was the biggest gainer in the poll. Michael Traugott, a political science professor at the University of Michigan, told Business Insider that Rubio's drop "just means he hasn't been generating much press coverage lately."
GOP strategist Liz Mair pointed out that Rubio's strong suit - personal-finance issues and foreign policy - took a back seat last week to topics like gay marriage, the Affordable Care Act, and the Charleston shooting.
"Probably for Rubio, it's better for people to focus on foreign policy," Mair told Business Insider on Wednesday.
Rubio might also be suffering from a mini-surge by former Florida Gov. Jeb Bush (R) and Trump, who both announced their presidential campaigns just more than two weeks ago.
"Rubio and Walker have lost a little altitude as Jeb Bush has righted his ship. Not sure I believe the Rubio numbers, but Jeb is looking like the front runner once again - at least until Aug. 6," Greg Valliere, the chief political strategist at Potomac Research Group, told Business Insider, referencing the date of the first Republican primary debate.
In any event, as The Huffington Post notes, Rubio's overall popularity has remained steady for the last several months, indicating that voters' attitudes toward him haven't shifted too greatly.