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"But if people view his efforts as a pattern of angling for political advantage -- even on the issue of immigration where in the span of three years his position has gone back and forth and back and forth again -- he's got some explaining to do," Benenson said. "And that makes his position harder."

Rubio is in a race to define who he is to a national audience before the opposition does. Tuesday's speech, following his high-profile address at the Republican National Convention in August, is his latest opportunity.

A Quinnipiac University poll last week showed that, nationwide, Rubio is relatively unknown to about 57 percent of the electorate. But of all the big GOP names polled, including former Gov. Jeb Bush and Romney-running mate Paul Ryan, only Rubio was viewed more favorably (27 percent) than unfavorably (15 percent).

That puts a political target on his back. Every changed position, every flip flop, will garner attention.

Democrats note, for instance, that his immigration stances have evolved on amnesty.

In 2010, for instance, he said during a Senate candidate debate on CNN that a "path to citizenship is basically code for amnesty." He wanted illegal immigrants to go back to their country of origin.

Now Rubio proposes a pathway to citizenship. He said it isn't "blanket amnesty" because the undocumented would have to pay fines, back taxes and pass a criminal background check. Before getting a shot at citizenship, they'd also heave wait until the borders are secure and then stand in line behind those immigrants who are legally here.

Rubio said he had to compromise. But he won't when it comes to more border security: a "real" fence, drone surveillance and better computer tracking of immigrants.

"What I'm not open minded about is that it has to happen and it has to be real -- because this is our last chance to get this right," Rubio said.

Under the current immigration proposal, an independent group would verify that the border is secure. The particulars, though, are unclear.

"We have a lot of work to do," he said. "There are very legitimate concerns by people in my party. They've heard these promises before."

If the legislation, which should be drafted by the end of March, doesn't meet his standards, Rubio said he'll oppose it.

The chances Rubio gets into specifics like that Tuesday night are slim.

If past rebuttals or State of the Union speeches are any guide, the words spoken Tuesday will have a short shelf life. The rebuttal given by Louisiana Gov. Bobby Jindal in 2009 was memorable for his awkward performance, not what he said.

Republicans expect far more of Rubio.

Scott Reed, a longtime Republican consultant and top strategist for the U.S. Chamber of Commerce, said Rubio's decision to take on immigration reform two years after he won office shows his "very strategic" way of seizing on a big issue the right way.

"Rubio is focused. He's not popping off on every issue, or running down to the floor of the Senate, making a fool out of himself," Reed said.

"Still, this is a tricky forum for Rubio," Reed said. "The president has the energy in the room, he's in front of 535 members of Congress and the members of the Supreme Court," Reed said. "Rubio won't have that."

But, on his current path, Rubio in four years has a good shot at standing where Obama will be Tuesday night.