One can gain a lot of understanding about the two major political parties when one examines the history of Vice Presidential selection by the major party Presidential candidates between 1960 and 2012, a total of 14 national elections.
If one looks at the Democratic Party, it is fact that ALL but one time, the Democratic Presidential nominee chose a sitting United States Senator to be his running mate as follows:
1960–Lyndon B. Johnson of Texas
1964–Hubert H. Humphrey of Minnesota
1968–Edmund Muskie of Maine
1972–Tom Eagleton of Missouri
1976–Walter Mondale of Minnesota
1980–Walter Mondale of Minnesota
1988–Lloyd Bentsen of Texas
1992–Al Gore of Tennessee
1996–Al Gore of Tennessee
2000-Joe Lieberman of Connecticut
2004–John Edwards of North Carolina
2008–Joe Biden of Delaware
2012–Joe Biden of Delaware
The only exception was 1984, when Walter Mondale selected Congresswoman Geraldine Ferraro of New York as his Vice Presidential running mate.
Also, after Tom Eagleton dropped out as the Vice Presidential running mate of George McGovern in 1972, due to having been revealed as having had psychiatric treatment, Sergeant Shriver, the former Peace Corps Director, head of the War On Poverty, Ambassador to France, and Kennedy in law, replaced him on the ticket.
All of the ten US Senators who ran for Vice President came to the national ticket as outstanding legislators with solid records of accomplishments, while Ferraro might be considered the weak link, the only real such case, for the Democratic national tickets. The only Senator who, in retrospect, might be considered not an ideal choice would be Edwards, for the personal life scandals that were revealed in later years.
Also, all of these Vice Presidential selections sought the Presidency after being chosen as a VP running mate, and Mondale, Gore, and Biden served notably as Vice President, all adding to the prestige of the office.
On the other hand, the Republicans had a very different scenario, as only four times out of fourteen did they select a United States Senator as their Vice Presidential choice for a national campaign, as follows:
1960—Henry Cabot Lodge of Massachusetts (former Senator 1936-1952)
1976— Bob Dole of Kansas
1988—Dan Quayle of Indiana
1992—Dan Quayle of Indiana
Three times, the Republicans selected state governors as their Vice Presidential nominees, as follows:
1968—Spiro Agnew of Maryland
1972—Spiro Agnew of Maryland
2008—Sarah Palin of Alaska
But most commonly, the Republicans for a total of seven times selected a member or former member of the House of Representatives, as follows:
1964—William E. Miller of New York
1980—George H.W. Bush of Texas
1984—George H. W. Bush of Texas
1996—Jack Kemp of New York
2000—Dick Cheney of Wyoming
2004—Dick Cheney of Wyoming
2012—Paul Ryan of Wisconsin
Out of these 14 cases, it is clear that Quayle, Agnew and Palin, in particular, stand out as horrible choices, and with the nation being burdened with nearly five years of Agnew and four years of Quayle in the Vice Presidency.
At the same time, Miller seems a nonentity who was chosen, and Cheney and Ryan, while competent, both stood out as particularly controversial selections, based on their public record in the past and the future as well.
Only Dole, Bush, and Kemp stand out as noncontroversial choices.
So it is clear that the Democrats have been much wiser in their Vice Presidential choices than the Republicans in the past half century!