Senate Republican Conference Chairman Lamar Alexander (Tenn.) took on the leader of his party tonight by opposing a GOP proposal to overturn an Environmental Protection Agency rule on clean air.

Alexander directly challenged an effort by the junior Senator from the neighboring state of Kentucky, the home of Senate Minority Leader Mitch McConnell. He did so without naming McConnell, his longtime friend, but he was blunt about how Kentucky’s pollution adversely affects his own state.

“Tennesseans admire much about our Kentucky neighbors — their bluegrass, basketball and distinguished United States Senators,” Alexander said in a statement announcing opposition. “But we don’t want Kentucky’s state income tax. And we don’t want Kentucky’s dirty air. ”

The display of independence follows Alexander’s announcement in September that he plans to step down from the No. 3 spot in the Republican leadership.

Republicans have been critical of the EPA, which they believe to be an overzealous regulator throwing up impediments to small businesses and preventing them from recovering from the economic recession. Sen. Rand Paul’s proposal would overturn the rule limiting the blowing of power plant pollution across state lines, and it is expected to receive a Senate vote Thursday.

Alexander also released a letter today urging support for legislation that would enact the proposed rule into law. Alexander and Sen. Mark Pryor (D-Ark.) are working together on the measure, which would give utilities an additional year to implement it.

“We are writing to invite you to join us in bi-partisan legislation that provides a better approach: enact the clean air rule into law, but give utilities one additional year to comply with it,” the letter said. “Our approach would provide certainty and cleaner air at the lowest possible cost to ratepayers.”

According to Alexander and Pryor’s letter, the rule is a revision of one established in 2005 under President George W. Bush’s administration.

“So this clean-air rule has been in place for six years,” Alexander said. “Many utilities have taken steps to comply with it. The pollution standards of the amended rule are about the same as those established in 2005 by the Bush rule. As an example of costs, the Tennessee Valley Authority, the nation’s largest public utility, tells us that complying with the amended rule will costs its ratepayers between $1 and $2 a month.”

Alexander and Pryor argued that overturning the clean air rule will “throw the matter back to regulators, lawsuits, courts and confusion.”