The Wisconsin primary that kicked off on Tuesday morning weighs heavily for a seesaw battle between two Democratic candidates, and for Republican front-runner John McCain.
Wisconsin would yield dozens of delegates, 74 for the Democratic Party and 40 on the Republican side. The polling stations closed at 9 p.m. EST (0200 GMT Wednesday).
New York Senator Hillary Rodham Clinton is counting on Wisconsin to stem her rival, Barack Obama, from continuing with his momentum built by several races in the month.
According to CNN statistics, the Illinois senator has accumulated 1,263 delegates, compared with the former First Lady's1,212. Previous polls in Wisconsin showed that the two candidates would stage another close race in the state's primary.
In Wisconsin, where industrial sections were badly hit, Clinton found her voice heard and responded well by rural and blue-collar voters in areas far from Obama's prime territory. She challenged Obama in TV ads aired in Green Bay, Eau Claire and La Crosse, jabbing his promise on "changes."
"There's a difference between speeches and solutions, between talk and action," she told a Monday rally. "I was raised to believe that actions speak louder than words."
Obama, for his part, is struggling to widen his lead in Wisconsin before the Ohio and Texas races, where Clinton has shown strong competitiveness.
He is eyeing on the state's capital, Madison, where thousands of students attend the University of Wisconsin, and expected to feed on young voters as he has done since the first caucuses in Iowa.
In addition, Obama released a new economic plan last week to appeal to voters in Green Bay, Oshkosh and other areas that are in need of industrial revival.
The influence African Americans would bring to Obama in Wisconsin is considered limited since they account for only 6 percent of the state's population. But the city of Milwaukee, which homes 75 percent of the black, is expected to overwhelmingly vote for Obama.
In the Pacific island of Hawaii, which would award 20 delegates to the nomination convention, a big win is expected to await Obama, who was born there to an American mother and a Kenyan father.
Wisconsin holds an open primary that allows registered voters to cast a ballot in either race, which could somewhat benefit Obama for his prevalence among independents and even some Republicans.
For Arizona Senator McCain, Wisconsin primary will serve as another test of his support among social conservatives, who still hesitate to stand behind the ready-to-be presidential candidate.
"If McCain wins by overwhelming margins, that would signal that the social conservative evangelical wing of the party has made their peace with his campaign," Ken Mayer from the University of Wisconsin-Madison told the USA Today.
McCain's remaining rival, former Arkansas governor and Baptist Minister Mike Huckabee, insists on staying in the race before McCain accumulates 1,191 delegates needed to win the Republican presidential candidacy.
McCain has so far won 881 delegates compared with Huckabee's 217, according to the CNN figure.