I offer some evidence in today’s Chicago Tribune:

Last week, Pennsylvania Sen. Arlen Specter was one of the most liberal Republicans in the Senate. Today, he’s the most conservative Democrat….

But party-switchers often change their votes as well as their labels.

The day after Republicans won control of the Senate in 1994, Sen. Richard Shelby of Alabama switched to the Republican Party. He had been a relatively conservative Democrat and had high-profile conflicts with President Bill Clinton, so the switch wasn’t a great surprise. But observers might be surprised to look back at what happened to Shelby’s voting record. According to the American Conservative Union, for eight years Shelby’s conservative voting percentage had ranged between 43 and 76. Even in 1994, as Shelby often found himself opposing the Clinton administration, the ACU gave him only a 55. But from 1995 to 2000, his ACU rating only once dipped below 90, and he scored a perfectly conservative 100 in 2000 and 2001….

In 2001, Sen. Jim Jeffords of Vermont left the Republican Party and became an independent. Conservatives said he was actually voting like a liberal Democrat. But that wasn’t quite right. Since he entered the Senate in 1989, his average ACU rating had been 27 — definitely the most liberal Republican, but not Ted Kennedy country. His ADA average was 58 — liberal for a Republican, but a long way from Vermont Democrat Pat Leahy. After the switch, Jeffords’ ACU rating started falling like GOP approval ratings: from 40 in 1999 to 29 in the year of the switch to 6, 10, 4, 8 and 4 during the rest of his tenure.

Specter says he won’t become a party-line Democrat, any more than he’s been a reliable Republican vote. But the evidence from previous party-switchers is that his votes will end up much more in line with his new party.