ST. LOUIS -- Republican Lt.  Gov. Peter Kinder continues to be chafed by his former partymate's cautious entry into the fight over health care.

On Monday, Democratic Attorney General Chris Koster -- who was a Republican until 2007 -- entered a brief in a Florida case involving 26 states seeking to overturn President Barack Obama's health care push.

Koster's brief challenges the most contentious provision of the new law -- the mandate for coverage -- but upholds the rest of it. In a letter to state House and Senate leaders, Koster said he personally favors expanding health insurance, even if his legal analysis offers a different route.

Kinder, appearing with Fox News' Greta Van Susteren on Monday night, said Koster's move is "is puzzling a lot of us."

"Ask anybody who knows him, my friend Chris Koster has finally developed political antennae," said Kinder, perhaps sardonically as he cited Koster's party switch a year before he ran statewide.

Kinder was pleased with Koster's argument that forcing individuals to have health care is beyond the authority of Congress. 

But "a lot of that brief is mush on other issues" Kinder said.

The lieutenant governor had been hoping that Koster would join a separate health care challenge that Kinder filed at the Rush Limbaugh federal courthouse in Cape Girardeau, Kinder's hometown.

That has always been an unlikely prospect; Koster would be loath to bring attention to the case filed by Kinder, who is widely expected to run for governor.

Instead, Koster weighed in on the Florida case, without actually joining it, and without actually saying he's against the president's health care plan.

Political antennae or no, it may be a shrewd move for an incumbent Democrat running for a new term in a state where the electoral landscape is a lot redder than it was when he won three years ago.