How much can Romney help Rubio?

WaPo reports:

Sen. Marco Rubio has been cultivating a relationship with Mitt Romney and his intimates, landing some of the 2012 Republican nominee�s top advisers and donors and persistently courting others as he readies an expected 2016 presidential campaign.

In a crowded field of contenders, the imprimatur of Romney could help clear Rubio�s path into the top tier. Since Romney announced in January that he would not run for the White House again, he and Rubio have had at least two lengthy phone calls in which Romney encouraged and mentored the 43-year-old Florida senator about the political landscape, according to a Romney associate.

Rubio and Romney have built a warm and trusting rapport, in contrast to the frostiness that exists between Romney and the two current GOP front-runners, former Florida governor Jeb Bush and Wisconsin Gov. Scott Walker. When Romney said in January that it was time to turn to the �next generation of Republican leaders,� it was widely interpreted as a swipe at Bush and a boost to a fresher face, such as Rubio.

What's the evidence of frostiness toward Walker?
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