Who is in the top tier of the Republican 2016 presidential race? The lineup has been changing over the last few months, and it appears to have changed again.

At the beginning of this year, the only constant in the race was that Jeb Bush was in a leading position in the RealClearPolitics average of national polls. Rand Paul scored well, Mike Huckabee scored well, Ben Carson scored well -- but there was no clearly discernible first tier of candidates.

Then, in late January, Scott Walker made a breakthrough appearance at the Iowa Freedom Summit. A top tier formed, and Walker immediately jumped into it. Since that time, Walker has been in double digits in 20 of 22 GOP polls included in the RealClearPolitics average. In the period from February 1 to mid-April, other candidates -- mostly Paul, Carson, and Huckabee -- did well occasionally, but the top tier was Bush and Walker.

Then came the Marco Rubio boom. Rubio declared his candidacy on April 13 and almost immediately jumped into the race's expanded top tier. In the period between his announcement and June 1, Rubio was in the top three in eight out of ten polls included in the RealClearPolitics average.

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