Most polling demonstrates that vice presidential selections ultimately do not directly affect the outcome of presidential elections.
It certainly did not help Mondale in 1984, the last time a presidential candidate picked a member of the House of Representatives to join his ticket. Nor did it help Gerald Ford in 1976, the incumbent president at the time. Nor can you say so about Al Gore in 2000, John Kerry in 2004, or John McCain in 2008.
These examples of the losing side all hold true, but it is no more clear that the winning side was helped by the vice presidential running mate selection, such as with Reagan picking George H.W. Bush in 1988, Clinton picking Gore in 1992, George W. Bush picking Dick Cheney in 2000, or Barack Obama picking Joe Biden in 2008.
You would have to look back to 1960 to find an example of a running mate making the difference in a presidential election. Had John Kennedy not picked the U.S. Senator from Texas, Lyndon Johnson, as his running mate he surely would not have become president.
Since vote-rigging allegations in Chicago always hung like a cloud over the results, and Richard Nixon had too much respect for the nation to challenge the results in court, the selection of a big-state running mate with the chance to help score a lot of electoral votes made a big difference.
If you believe the stories about how much Kennedy loathed Johnson, then you can see the motive behind his selection.
Therefore, do not look to Paul Ryan to win this election, but you should look to see how he can sell Mitt Romney’s message of fiscal sanity, a duty for which he is quite well suited.