Washington - Senator Barack Obama won the Vermont primary, exit polls showed, capturing the first of four states up for grabs in balloting Tuesday to determine the Democratic presidential nominee. On the Republican side, Senator John McCain, 71, captured Vermont, further tightening his grip on the centre-right party's nomination for the November 4 presidential election. Major television networks also projected McCain would win in Ohio.
Obama, 46, defeated Senator Hillary Clinton, 60, in Vermont, and and the campaigns for the hotly contested centre-left Democratic nomination were awaiting the results in Rhode Island, Texas and Ohio.
Polls show Clinton and Obama in tight races in Texas and Ohio, the two most important states in Tuesday's elections.
Obama has recently emerged as the clear Democratic frontrunner; if Clinton loses in both Ohio and Texas, the former first lady would probably fall insurmountably behind.
Obama was expected to win in Vermont, and pre-polls showed that Clinton had a sizeable lead in Rhode Island. Pre-election polls in Texas showed a dead heat, and Clinton had held a small lead in Ohio.
Heading into Tuesday's voting, Obama had won 11 consecutive state contests, establishing himself as the frontrunner in the delegate count and building up momentum. Meanwhile, Clinton's campaign has run short of money and struggled to combat her rival's growing image of inevitability.
Former president Bill Clinton has said that his wife must win in Texas and Ohio to keep alive her hopes of becoming the first female president. Hillary Clinton has vowed to go forward with her campaign regardless of Tuesday's outcome.
Some Democrats, however, have urged Clinton to drop out of the race so that Obama can begin focusing on McCain in the presidential election without having to devote campaign resources to continue duelling with Clinton.
McCain effectively sealed up the Republican nomination weeks ago and has increasingly refocused his presidential campaign on the November general elections. He has begun making the case for keeping US soldiers in Iraq - a position Clinton and Obama oppose.
"I feel really good not only about the primary today but also about what's possible for us in Texas come the fall," Clinton said in last-minute campaigning in Houston.
The Clinton and Obama camps have become nastier in recent weeks, exchanging barbs over campaign tactics and highlighting their differences over the North American Free Trade Agreement (NAFTA), enacted during Bill Clinton's administration.
While the trade accord has benefited border states like Texas, it is unpopular in Ohio, which has suffered from a loss of manufacturing jobs. Both candidates say that NAFTA needs to be changed to protect US workers, and both have proclaimed willingness to pull out of the agreement if it cannot be renegotiated.
The Clinton campaign has drawn attention to allegations that one of Obama's economic policy aides met with a Canadian official to calm fears that he would ditch NAFTA, saying Obama's comments were just political rhetoric.
Obama responded by saying that Clinton was pedaling a false story of "contradictions and winks and nods that has been disputed by all parties involved." He also pointed out that Clinton and her husband pushed NAFTA through Congress in 1993.
Clinton aides have raised Obama's relationship with a developer in Chicago, Antoin Rezko, now on trial for exploiting political relationships with the current governor of Illinois to obtain kickbacks on state contracts.
Obama charged that Clinton was throwing the "kitchen sink" in an effort to rescue her bid for the White House.
Clinton has also turned to comedy to try to swing momentum and alter her no-fun image. She appeared on NBC's Saturday Night Live and Comedy Central's The Daily Show, at times poking fun at herself.
Clinton has touted her experience in the White House and longer tenure than Obama in the Senate to argue that she is better prepared for the presidency, and usually says she will be ready to take the helm on "day one."
Obama's message of change has caught on with voters and propelled him to the front of the race, bucking Clinton from her dominating position early in the campaign.
A recent television ad from Clinton's campaign shows sleeping children, then asks whom viewers would want to be in the White House when the phone rings at 3 am. Obama quickly aired a similar ad, likewise showing sleeping children and asking Democratic voters whether they want someone in the White House who voted to authorize President George W Bush's use force in Iraq.
Clinton voted for the October 2002 measure, but says if she knew how the Bush administration would bungle the post-invasion planning, she would have voted against it.
