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Conventional wisdom holds that veeps don't determine elections; voters vote the top of the ticket.

But has there ever been a vice-presidential pick whose ideas/ideology so overshadowed the presidential nominee's? Has any presidential nominee ever essentially run on his vice-president's ideas?

Ronald Reagan almost picked Gerald Ford to mollify moderates in 1980. But he balked when Ford insisted Henry Kissinger return as Secretary of State. Reagan was his own man politically; he knew what he was about and so did everyone else. Not sure you can say the same about Romney.

In some ways, Richard Nixon was forced on Dwight Eisenhower in 1952 by the GOP's staunch anti-communist conservatives. They did not entirely trust Eisenhower, who wasn't even a Republican until 1952. And Ike was uncomfortable with Nixon's slash-and-burn tactics-and nearly dumped him. The ticket won, though, because America truly liked Ike. Not sure you can say the same about Romney.

Paul Ryan, then, is an unusual pick historically. Romney is in uncharted waters as the nominee, having a running mate who some will say overshadows him ideologically. How Romney demonstrates convincing leadership of his own ticket may be one of the key tests of his candidacy in this campaign.