THE NATION - When moderates feel lost in the GOP - A Missouri state senator's decision to switch parties highlights regional frustration with social conservatives.

Not only did Koster join the marginalized minority party in Missouri, but he did so with a thundering speech that lambasted his former colleagues as ignoring the needs of their constituents and slavishly following the dictates of "religious extremists."

The former prosecutor denounced several Republican positions he had once supported, such as steep cuts in Medicaid coverage and subsidized family-planning programs.

But Koster reserved his harshest criticism for GOP efforts to overturn a voter-approved constitutional amendment that protects embryonic stem-cell research in Missouri.

"The Republican desire is to criminalize early-stage stem-cell research in our state," Koster said in a speech he repeated three times as he hopscotched across the state. "Go to Boston for your Nobel Prize; come to Missouri for your leg irons. And the Missouri Republican Party not only tolerates this lunacy, but embraces it," Koster said.

Days later, one of his staffers updated his website -- by deleting a photo of Koster shaking hands with Vice President Dick Cheney.

Koster's decision stunned Republicans here in his district just south of Kansas City and across this quintessential swing state. "There's no precedent for it in the state of Missouri," said GOP consultant Paul Zemitzsch.

But the move sounded like deja vu just across the state line in Kansas.

Three prominent Kansas Republicans moved into the Democratic column in late 2005 and 2006, voicing similar concerns about the influence of social conservatives. One of those defectors was elected attorney general. Another -- who once chaired the Kansas Republican Party -- now serves as lieutenant governor.

Political analysts don't expect a cascade of party-swappers in Missouri. As political scientist David Webber put it: "I'll be darned surprised if anyone follows [Koster's] example."

But they say the move does point at how effectively social and religious conservatives dominate the Republican Party across several Midwest states -- and how frustrating that can be to self-styled moderates who would prefer to focus on economic issues.

"That's been true for a decade," said Webber, a professor at the University of Missouri at Columbia.