Donald Trump’s selection of Mike Pence as his running mate shows an interesting difference between Republicans and Democrats. The Democrats will almost always pick Senators for Veep. Republicans are most apt to pick Governors, like Pence is now; leading members of the House of Represenatives, like Pence was until four years ago; and members of former presidential administrations, which doesn’t quite work in the current political environment due to George W Bush’s reputation.
Since 1980, the GOP has gone with one Senator for Vice-President: Dan Quayle. The other Vice-Presidential candidates included a Governor, three representatives turned Cabinet/ Cabinet-level members of previous administrations (Papa Bush, Kemp, Cheney) and with Paul Ryan, one sitting representative. As a Governor, with a notable stint in Congress, Pence has a mix of the records of Palin and Ryan.
Democrats have consistently chosen a Senator for the #2 spot since ’88, with Bensen, Gore, Lieberman, Edwards and Biden. This still applied when the top of the ticket was a sitting or former Senator.
The only time Republicans had two Senators, former or current, on the ticket was 1960, when Vice-President and former California Senator Richard Nixon chose former Massachusetts Senator Henry Cabot Lodge Jr to be his running mate. Democrats had two-Senator tickets (counting Vice-Presidents who came from the upper chamber) in 1948, 1960, 1964, 1968, 1972 (for 19 days before Eagleton was replaced), 2000, 2004 and 2008.
Part of it could be a desire of the parties to avoid mistakes made in the past. The only Senator recently chosen for Vice-President by the Republicans was Dan Quayle, who has since been widely ridiculed. The only non-Senator recently chosen by Democrats for the office was Geraldine Ferraro, something that’s now considered a poor decision. The main reason Ferraro was on the ticket was that Mondale wanted someone who wasn’t a white man to be his running mate, so his options were limited. The alternatives to Ferraro—a third-term congresswoman—were San Francisco Mayor Dianne Feinstein, Los Angeles Mayor Tom Bradley, San Antonio Mayor Henry Cisernos and Kentucky Governor Martha Collins, who had served less than an year in the position. If there had been a female Democratic Senator at that point—if Elizabeth Holtzman had won her 1980 New York Senate race by one point rather than losing by that margin—Mondale would have been happy to run on a ticket with her.
It could also reflect differences between the parties. Republicans haven’t been terribly fond of Washington, and especially the culture of the Senate, so they’re less inclined to go with politicos who are seen to be a part of that atmosphere. Democrats usually like the idea of big government. Republicans prefer executive power, which allows for Governors and administration members. They also seem to prefer the House to the Senate.
The selection of a candidate can also influence subsequent events. If Republicans look at George HW Bush as an adequate Vice-President, they’re more likely to pick successors made from the same mold. If all the previous running mates have been Senators and that usually worked out well for the Democrats, the Presidential candidates would be less inclined to broaden the long-list to include those who served in previous administrations.
Republicans had held the White House more often, which made it easier to choose from presidential appointees. That’s changed in recent years, as the George W Bush administraiton was decidedly unpopular, while Barack Obama became the second twice-elected Democratic President since FDR. Labor Secretary Tom Perez and HUD Secretary Julian Castro are considered potential running mates for Hillary Clinton, as is Congressman Xavier Becerra of California (#4 in the Democratic leadership) although part of that may be that the only Hispanic Democratic member of the Senate is currently under investigation.