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Donald Trump, Hillary Clinton lead their fields nationally

Donald Trump and Hillary Clinton continue to lead their respective fields in the presidential race, according to an NBC News/SurveyMonkey Weekly Election Tracking Poll released Tuesday.

The poll found that nearly half of Republican voters, 48, percent back Trump, over 20 percentage points more than the 27 percent who support Ted Cruz. Eighteen percent said they support John Kasich.

A majority of Republicans, 57 percent, said Trump should win the nomination even if he only wins a plurality of the required 1,237 delegates and not a majority. More than a quarter said Trump shouldn't win the nomination and 14 percent said they were unsure. Among Republicans who back Cruz or Kasich, a majority said Trump shouldn't be the Republican nominee if he fails to win a majority of delegates before the Republican National Convention this July.

More than half of GOP voters said they would be satisfied if Trump and Clinton make it to the general election, but nearly a third said they would seriously consider supporting a third-party candidate.

The Democratic race, meanwhile, has tightened over the last week. Nearly half, 49 percent, of Democratic voters back Clinton, down from 53 percent last week. The survey found 43 percent now back Bernie Sanders, up slightly from 41 percent last week.

While last week's poll showed Clinton leading among men 51 percent to 45 percent, Sanders now has the edge with 47 percent to 45 percent support. Clinton has also lost some support among key racial groups. Support among white voters for Clinton has dropped 3 percentage points, it has dropped 5 percentage points among Hispanics and it has dropped 4 percentage points among black voters.

The survey comes ahead of Wisconsin's primary next Tuesday.

The poll surveyed 6,521 adults, including 5,741 registered voters, between March 21 and 27.