For balancing purposes, McCain needs to do two things: a) choose another conservative who hasn't irked the Republican base as he has - so as to unite the party, b) someone with stronger credentials on the economy.
The Democrats will use, to some success, the infamous 2005 quote where McCain said he didn't know much about the economy. He could use his vice presidential candidate to help fight this.
I will start by, irrespective of the requirements, naming two pet possibilities:
Sen. Elizabeth Dole (R-NC)
Fmr. Gov. Jeb Bush (R-FL)
Why? Because every Republican ticket since 1952, save one, has had a Nixon, Dole or Bush on it and I am amused by such historical oddities.
Dole would probably satisfy all of the requirements I set out. She is a solid conservative (ACU rating of 96) and, as a former labor secretary, has knowledge of the economy. She is also a classy lady who would warm voters hearts and certainly not lose votes. Her main disadvantage is that she is the same age as McCain and he'll probably want to balance the ticket with age as well. Moreover, she is running for re-election this year (and will be nominated in May) so that could make her an awkward veep candidate, though didn't stop Gore from choosing Lieberman in 2000.
Bush satisfies the conservative and economic qualifiers but, by virtue of his surname would probably cost McCain votes especially in that he would aid the Democrats in painting the Arizonan as Bush III.
Now on to others, (in alphabetical order):
Sen. Sam Brownback (R-KS)
Brownback is certainly a conservative (ACU rating of 87), and even more especially a social conservative. After abandoning his ill-fated bid for the presidency he endorsed McCain which might be viewed as a plus. However, his support of McCain's immigration reform doesn't make him a very good candidate for bringing in the base. Moreover, he doesn't have much economic experience.
Sen. Richard Burr (R-NC)
An early supporter of McCain, Burr is a strong conservative (ACU rating of 92). He is young and has some experience but little focused on the economy. He voted on both sides of the immigration bill but unlike McCain and Brownback was not an unabashed supporter of it.
Sen. Tom Coburn (R-OK)
Though he is certainly a solid conservative (ACU rating of 100!), this former physician has been to the far, hard right on a number of issues which would likely turn off McCain. Moreover, he lacks experience on the economy.
Gov. Charlie Crist (R-FL)
Crist's endorsement of McCain on the eve of the Florida primary may have pushed the Arizonan over the top and that win in Florida is what sealed the nomination, thus some favours are certainly owed here! However, Crist doesn't have a strong record as a conservative and Florida is likely to vote Republican with or without him on the ticket.
Sen. Lindsey Graham (R-SC)
A fairly solid conservative (ACU rating of 83), Graham was a strong backer of immigration reform and doesn't have a lot of depth on economic issues. He doesn't balance the ticket.
Fmr. Gov. Mike Huckabee (R-AR)
Huckabee was for a long time considered a favourite for a VP candidate, however, McCain is distrusted by some economic conservatives as is Huckabee. He would not be a good fit for McCain.
Gov. Jon Huntsman (R-UT)
Huntsman, a Mormon and governor of Utah, bucked many of his fellow members of the LDS church and fellow Utahans by packing John McCain over fellow Mormon Mitt Romney. He has foreign policy experience (a former ambassador) and economic experience (a former deputy U.S. trade representative and a successful governor overseeing a booming economy). He is young and conservative and could be a surprise choice by McCain.
Gov. Bobby Jindal (R-LA)
A former congressman and freshly elected governor, Jindal is a solid conservative (ACU rating of 92) and particularly a strong social conservative. He is only 36 and thus barely eligible for the vice-presidency (one must by 35) but has a remarkable amount of experience for his age. He was in the Louisiana cabinet at 25, ran a
national commission on medicare at 27, was named president of the University of Louisiana at 28 and was appointed as an Assistant Secretary in the federal health department at 30. He has had tremendous success in health care issues and the Democrats will be trying to make that a major issue in the election. Moreover, his multiculturalism could be the Republican answer to Barack Obama. Like Huntsman, he is another darkhorse.
Gov. Tim Pawlenty (R-MN)
This fellow seems to be the darling choice of the media. Pawlenty was the strongest and steadiest supporter of McCain through the dark period of 2007 when the latter's campaign seemed to be over. However, he is not a strong conservative and therefore would be a poor choice for ticket balance.
Fmr Rep., Int'l Trade Amb. and White House Budget guy Rob Portman (R-OH)
Portman isn't a household name, but he has lots of experience. He was a congressman for 8 years (getting an ACU rating of 88 in his last year) and served for 2 years as a junior member of the Bush cabinet (first as International Trade Representative [essentially the U.S. trade minister] and then as White House budget director [eseentially a finance minister in a minority government, with less leaway]). He is highly regarded among Washington insiders and is attractive and charismatic. Moreover, Ohio will be a key state (as it was in 2004) and he could help keep it in the Republican column.
Gov. Sonny Purdue (R-GA)
Purdue is only 10 years younger than McCain, has no national profile, no international experience and no tremendous economic record to present. Why he has appeared on some lists as a potential pick escapes me.
A former congressman from and governor of Pennsylvania, the current Democratic governor of the swing state (which went Democratic by small margins in 2000 and 2004) said Ridge would push the state into the Republican column. Though he is not a solid conservative (he got a 61 rating from the ACU his last year in congress), he is well regarded for his stewardship of the terror file by conservatives and is a key Bush ally. He has supported McCain since day one and seems close to the Senator and delivering Pennsylvania would almost guarantee a victory for the Republicans.
Gov. Mike Sanford (R-SC)
A solid conservative (ACU rating of 92) and a compatriot of McCain in the anti-pork spending crusade, Sanford could be a good way to balance the ticket and to please McCain.
Fmr. Sen. Fred Thompson (R-TN)
Thompson is a solid conservative and an old friend of McCain's from the Senate and from the latter's 2000 presidential run. Had he not run for president, or run and done better, I would imagine him as a frontrunner for the McCain VP nod. However, owing to his disappointing performance when he was running to be at the top of the ticket, I dought he would be tapped to run at the bottom.
Fmr. Rep. J.C. Watts (R-OK)
Watts, one of view Black Republicans ever elected to national office, is a strong conservative (ACU rating of 96) and was always considered a rising star before he opted to honour his committment to only serve three terms. Still relatively young, he could be a good fit for McCain.
No way Jose: The following names have been mentioned but would never be pick
First, McCain is on the record saying that he doesn't expect that the
party would nominate a pro-choice VP candidate. That rules out the
following who are sometimes mentioned:
Sen. Susan Collins (R-ME)
Fmr. Mayor Rudy Giuliani (R-NY)
Sen. Joe Lieberman (I-CT)
Fmr. Gov. Mitt Romney (R-MA)*
Fmr. Secretary of State Colin Powell (R-NY)
Sen. Olympia Snowne (R-ME)
Fmr. Gov. Christie Todd-Whitman (R-NJ)
Secretary of State Condi Rice (R-CA) may or may not be pro-choice, but her closeness to the Bush administration and unknown performance is true political arena would prevent her from being chosen.
*- Although Romney is now pro-life his previous positions on this and
many other issues would violate the "do no harm" principle
The likely picks, in order of likelihood in my view:
NOTES: ACU ratings for the most recent year of service have been used; the list contains all names I have been able to find on veep lists + some of my own (Thompson, Dole and Watts).
UPDATE: An earlier version of this post included Tom Ridge as the #4 most likely pick, however I have since learned he is pro-choice and thus not so likely. (h/t Jonathan Martin)