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Somewhat fittingly, many recent party cross-pollinations have direct ties to conservative standard-bearer President Ronald Reagan — himself a former Democrat. His son Ron Reagan, formerly a liberal commentator on MSNBC, was given a prominent speaking role at the 2004 Democratic National Convention, for example.

In 1996, Democrats also managed to snag Jim Brady, Mr. Reagan’s former press secretary and a gun-control advocate who was partially paralyzed in the 1981 attempted assassination of the president.

In 1984, another Reagan associate — his U.N. ambassador and former Democrat Jeanne Kirkpatrick — gave a rousing speech at the Republican convention, in which she said that the party she left wanted to “blame America first.”

“It certainly sometimes makes for good entertainment and headlines,” Mr. Alpert said of the practice. “Sometimes, controversy that they can control is a good way to get that.”