Donald Trump and Hillary Clinton continue to lead nationally in their parties, leading up to the South Carolina primaries and the Nevada caucuses.

Trump leads among Republicans, with 38 percent support, followed by Ted Cruz with 18 percent and Marco Rubio with 14 percent, according to a NBC News|SurveyMonkey online poll released Tuesday. Ben Carson holds 8 percent support, while Ohio Gov. John Kasich rises to a two-month high of 7 points. Only 4 percent support former Florida Gov. Jeb Bush.

Fifty-six percent of Republican voters believe Trump will be the GOP nominee, and Trump has also eaten into Ted Cruz's margin among white evangelicals, shrinking the Texas senator's lead to nine points in that group.

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Clinton's lead over Bernie Sanders among Democratic and Democratic-leaning voters is largely unchanged, at 50 percent, down just one percentage point since last week, but it's still a 10-percentage-point lead. A third of Democrats think Sanders will win the nomination, but two-thirds think Clinton will clinch it.

Clinton and Sanders have been wooing minority voters as voting moves to states with greater racial diversity than New Hampshire and Iowa. This poll found that Democratic and Democratic-leaning black millennial voters support Clinton 64 percent to 25 percent, just a little less than her support from older black voters' support for her, 73 percent to 16 percent among those over 35.

White millennial Democratic voters prefer Sanders, by a margin of 75 percent to 22 percent.

This comes just days before Nevada's Democratic caucuses and South Carolina's Republican primary on Saturday.

The poll surveyed 13,129 adults between Feb. 8 and Feb. 14 with 11,417 registered voters and a 1.1 percentage point margin of error.