SPRINGFIELD - U.S. Senate candidate Mark Kirk's changing positions on a "cap and trade'' environmental bill are inspiring angry boos from some fellow Republicans and accusations of flip-flopping from Democrats.

Democrats on Friday challenged Kirk, who's serving his fifth term in the U.S. House, to explain why he now opposes a measure that he said three months ago was good for national security.

``It appears that he wants to win an election and he's willing to do that even if it means we have to keep fighting over foreign oil,'' said Jill Morgenthaler, Illinois' former director of homeland security and an unsuccessful Democratic candidate for Congress in 2008.

Kirk was one of just eight House Republicans to vote for the bill in June. His position infuriated some conservatives, who consider the measure little more than an energy tax that will hurt manufacturers, the coal industry and consumers.

He has encountered significant hostility from conservatives over that vote and other issues, such as his support of abortion rights. He was greeted with a shower of boos when he addressed a Republican rally last week in DuPage County.

Kirk told the crowd that he supported the cap-and-trade bill because it was the right thing for his congressional district in Chicago's northern suburbs, but that he would oppose it as a senator representing the entire state.

``That's because we are a manufacturing, agriculture and coal state,'' he said to applause.

It was the latest in a series of changing statements on the bill, which would cap the pollution that companies can release but then allow them to buy and sell permits for more emissions.

When he voted for the bill, Kirk said it would help reduce the chances of more American troops being sent overseas to fight wars over oil. He also said the country needed to fight global warming and that a similar cap-and-trade program for reducing acid rain had worked well.

After launching his Senate campaign in July, Kirk said he ``got the message'' from Republicans who were angry over his position. But he didn't flatly say then that he was wrong to support the bill or that he would oppose it in the Senate.

Kirk and his campaign wouldn't answer questions about the issue Friday. Campaign spokesman Eric Elk released a statement saying that as senator, Kirk would look for ways of reducing dependence on foreign oil without killing thousands of Illinois jobs.

Illinois conservatives differ on whether Kirk's new promise to oppose the plan can repair his relationship with the right.

In the Republican primary, he faces challengers with limited money and name recognition. He might win the primary even with a significant number of conservatives angry at him. The bigger question may be whether he needs a united GOP to beat the Democratic nominee in the fall.

John Tillman, head of the Illinois Policy Institute think tank, said he's glad to see Kirk studying the cap-and-trade issue and changing his position. He argues that other conservatives should welcome Kirk's willingness to reconsider an issue so important to them.

``I think he's got a shot at getting those voters back. He's not going to win without them,'' Tillman said.

Fran Eaton, editor of the Internet site Illinois Review, said she's hearing from conservatives who don't trust Kirk. They worry that he's simply saying what they want to hear, but once in office, would turn to the left.

``I think he hopes to repair the damage, but the people I talk to are feeling insulted that we're just supposed to believe anything and everything,'' Eaton said.

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Associated Press Writer Deanna Bellandi in Chicago contributed to this report.