Poll: Rubio and Clinton top 2016 contenders in Iowa, nationally

But Rubio's advantage is tenuous relative to that of Clinton. Nationally, Clinton would have the support of 58 percent of primary voters, versus 19 percent for Vice President Biden, 8 percent for Sen. Elizabeth Warren, and 3 percent for New York Gov. Andrew Cuomo. 

Clinton polls even better in Iowa, where 68 percent of primary voters say she would be their choice. Biden earns 21 percent, and no other candidate got above 2 percent. Nine out of 10 Iowa Democrats view her favorably.

Clinton would also have the early lead in a hypothetical showdown with Rubio, leading the Florida Republican by 8 points nationally. But other Republicans poll better against Clinton: She leads Ryan and Bush by 6 points, and Christie by 4 points. Biden would lead Rubio by 5 points, Ryan by 4, Bush by 3 and tie with Christie.

Both Clinton and Rubio have been evasive on the topic of a presidential run.

"I really believe that if I do the best job I can in the Senate, then in a couple of years I'll be in a position to make a decision about whether I want to run for reelection, leave politics and give someone else a shot, or run for some other position," Rubio said at a BuzzFeed forum on Tuesday.

On Wednesday, it was announced Rubio would give the rebuttal to President Obama's State of the Union address, offering a national platform to the freshman lawmaker.

Clinton, who left the Obama administration late last month, has said she plans to enjoy retirement, but she is already fueling speculation about a reentry into politics with a new website.

Clinton told CNN last month she had "absolutely no plans to run" but then emphasized she felt healthy enough for any challenge.

"I am lucky because I've been very healthy. I feel great," Clinton said. "I've got enormous amounts of energy that have to be harnessed and focused, so I'm very fortunate. I'm looking forward to this next chapter in my life, whatever it is."

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