Wisconsin Gov. Scott Walker

Walker crashed and burned as a GOP candidate for president.

At the beginning of the year, he was in the top tier. After the second debate in September, he was gone.

Yet Walker remains someone widely respected by Republicans across the country. He also has a reputation as a conservative fighter and would be a welcome surrogate for any campaign.

If Walker were to choose to endorse Rubio, for example, it could give the Florda senator a boost from the right as he is battling Cruz for conservatives as well as the remaining establishment candidates.