When choosing a running mate, Mitt Romney will consider many factors: Can this person offer a bump in the polls? Will he or she excite the base? Could this person take the reins if necessary? Will we get along?
He will also weigh the candidates' ideological moorings. Let's take a look at where the candidates stand based on their voting records in public office.
For the answer, we dive into the informative--albeit imperfect--world of congressional scorecards. Advocacy groups and news outlets commonly rate lawmakers based on some standard relevant to the organization, which helps us determine where each contender falls on the ideological spectrum.
In the Senate, Rob Portman of Ohio, Marco Rubio of Florida, Kelly Ayotte of New Hampshire and John Thune of South Dakota commonly appear on lists of possible running mates. Scorecards that measured voting records in 2011 suggest that among these, Rubio had the most conservative voting record, and Portman the most moderate. Ayotte and Thune came in somewhere in between.
Florida Sen. Marco Rubio
Across the board, Rubio scores the highest in ranking studies conducted by conservative groups, in part because of his limited voting record in Congress. In reports filed by Heritage Action, The Club for Growth and The American Conservative Union, each group placed the newcomer in their top tier of conservative members.
Heritage Action, which uses one of the most stringent and specific ranking formulas, judges lawmakers not just for how they vote, but by their involvement in the process. With a 96 percent Heritage Action rating, Rubio ranks as the top four most conservative members of the Senate.
According to the National Journal 2011 Vote Rankings, which lists members from the most liberal to most conservative, Rubio still lands far to the right. The magazine in 2011 ranked him as the 13th most conservative member of the Senate, just three notches below conservative stalwart Jim DeMint.
Ohio Sen. Rob Portman
Chris Redfern, the chairman of the Ohio Democratic Party, recently said said that Portman represented " the extreme fringe of the Republican Party," but an examination of his voting record over his nearly 15 years in Congress does not live up to that rhetoric. While Portman's record makes it clear that he is no liberal, his record puts him more in line with the pragmatic wing of the party.
National Journal's analysis of the Senate in 2011 put Portman just two places ahead of Indiana Sen. Richard Lugar, who in May lost a primary battle to tea party-backed candidate Richard Mourdock. The magazine named Portman the 35th most conservative senator, far from the "fringe" members. (If you examine his entire record, however, Portman is no Lugar: He holds an average rating of 87.92 percent by the American Conservative Union over his career, putting him more than 10 percentage points ahead of the Indiana senator.)