Barack Obama and John McCain won the so-called Potomac Primaries, Maryland, Virginia and Washington, D.C., locking in McCain's already comfortable margin over Mike Huckabee and giving Obama a surge of momentum heading into competitive primaries with Hillary Clinton in Ohio, Texas and Pennsylvania.
On the Democratic side, the voters are siding more decisively with Obama, but the count of delegates that each candidate has won to the nominating convention is in a virtual dead heat.
All three candidates with a chance to win their party's nomination and vie for the White House Clinton, McCain and Obama have strong, but different credentials on environmental issues.
McCain doesn't have a stellar lifetime voting record for environmental issues, but among his maverick tendencies has been a passion for fighting global warming. He authored the first Senate legislation, in 2003, that would have regulated carbon dioxide pollution.
Clinton and Obama both have pledged to enact such a cap-and-trade regulation, and make it far stronger than the one McCain supported in the past. They have different energy plans that both call for huge investments in renewable and alternative energy sources.
For a detailed profile of each candidate, see Green Your Vote 2008.
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