Scoring the Romney running mate choices: Portman the moderate option, Rubio the most conservative

— -- 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.)

Wisconsin Rep. Paul Ryan

The Republican budget proposals drafted by Wisconsin Rep. Paul Ryan since he became chairman of the House Budget Committee have boosted his stock tremendously among conservatives, but National Journal's analysis found that, compared to his House colleagues, Ryan doesn't rank in the top 100 conservatives in the chamber.

National Journal's report labeled him the "150th" most conservative House member in 2011.  Meanwhile, his ACU rating dropped 16 percentage points in 2011 to 80 percent, although he retains a high lifetime rating of 91.96 percent.

The Governors

There a chance Romney will look beyond Washington to make his pick. Unfortunately, the market for scorecards that measure the record of America's governors isn't nearly as robust as it is for Congress.

The Fiscal Policy Report Card on America's Governors, conducted every two years by economist Chris Edwards of the libertarian Cato Institute, is one measure to judge the state executives. Edwards is still preparing the 2012 report, which will include grades for New Jersey Gov. Chris Christie and Virginia Gov. Bob McDonnell, so his 2010 database is the most recent available. In that report, which measures governors' tax policy record and awards a letter grade based on a bell curve, two of the potential VP choices received top marks. Louisiana Gov. Bobby Jindal and former Minnesota Gov. Tim Pawlenty were two of just four governors to receive an 'A' grade in the 2010 biennial report. Jindal scored second-highest in the nation with a score of 71, with Pawlenty just behind him with a score of 66.

Although Edwards' 2012 report is due out in September, he offered some hints as to how to other governors rumored to be on Romney's list of possible running mates would fare. In his preliminary research, he said, things might not look so good for McDonnell, but Christie could score high.

"He's not a staunch fiscal reformer," Edwards said of McDonnell. "He's an establishment guy. He wants to get along. He wants to be loved. And that's established in his record. Chris Christie is the exact opposite."

Rubio -- 97 percent

Rubio sought the approval of the club early in his political career, looking to the network of free-market conservatives to bolster his credentials and his support when few knew of him.

Portman -- ranked 29th in 2011 with an 80 percent score. That is also his lefetime average since the Club began using the scorecard.

Thune -- 20th in the Senate, 89 percent in 2011 83 percent lifetime

Ayotte -- ranks 8th in the Senate, 98 percent score in 2011, her first year in the chamber

Ryan - 101 in the House with a 73 percent score in 2011 and an 88 percent lifetime average