Illinois Senator Barack Obama and Arizona Senator John McCain won their parties' presidential primaries in Wisconsin, according to projections by all major television networks.
Obama, vying to become the first African American president, has been locked in a tight contest with Senator Hillary Clinton to secure delegates to the centre-left party's nominating convention in August.
Obama has picked up wins in more states than Clinton and has recently opened a small lead in the delegate count, but victories by the former first lady in large states and close contests elsewhere have kept the race close.
With 52 per cent of precincts reporting, Obama had 56 per cent of the vote to Clinton's 43 per cent.
The two candidates have spent the last week shuttling between Wisconsin, a Midwestern state between Lake Michigan and the Mississippi River, and the looming big-state contests on March 4 in Ohio and Texas. Both are considered must-win states by the Clinton campaign.
The Democrats did not take a break from the campaign trail to watch Tuesday's results, with Obama speaking to a rally in Houston as he greeted the Wisconsin win and urged voters in Texas to go to the polls on March 4.
"I am grateful to the people of Wisconsin for their friendship and their support and their extraordinary civic pride," Obama said. "In Wisconsin, when you go to vote its 5 degrees (Fahrenheit, or minus-15 degree Celsius) outside, but that has not discouraged people from Milwaukee to Eau Claire from casting their ballots."
Obama has opened up a lead of nearly 70 delegates over Clinton in the overall battle to win the 2,048 needed to capture the Democratic nomination. At stake Tuesday were 94 pledged delegates.
McCain is his party's clear frontrunner but must still pick up delegates to formally secure the nomination to face the Democratic winner in Nov 4 elections. He was challenged in Wisconsin by former Arkansas governor Mike Huckabee, who has gained support among social conservatives and picked up wins mainly in Southern states.
McCain had 54 per cent of the Republican vote to Huckabee's 38 per cent, with 52 per cent of precincts reporting.
"My friends, I'm not the youngest candidate, but I am the most experienced," McCain joked to great applause at a rally after citing his military and foreign policy credentials and taking a shot at Obama's theme of political change, referring to him as an inexperienced candidate.
Experience has become a major theme in the race, with Clinton telling crowds that she can lead the country while stressing Obama's relatively short time on the national stage, having joined the US Senate only three years ago.
A Democratic caucus was scheduled later Tuesday in Hawaii, where Obama was born, and primaries were held in Washington state, where a caucus was held in both parties earlier this month but Republican delegates remained at stake.