Fewer people think Donald Trump will eventually be the Republican nominee, even though Trump still holds a double-digit lead in the latest national poll.

According to a new NBC News/Survey Monkey poll, only 42 percent believe Trump will be the nominee, a huge 20 point drop from last week. Another 31 percent believe Sen. Ted Cruz will get the nomination, while 18 percent say the same about Sen. Marco Rubio.

Both Cruz and Rubio have gained 10 points since Iowa's caucuses last week.

While Trump's second-place finish in Iowa may have shaken people's faith in Trump, he's still up 15 points over his nearest competitor. He leads Cruz 35-20, although that's a 4 point drop from the last poll, while Cruz held steady.

Rubio crept up 4 points to 12 percent over that same time period.

Retired neurosurgeon Ben Carson sits in fourth place with 7 percent, while former Florida Gov. Jeb Bush, New Jersey Gov. Chris Christie and Ohio Gov. John Kasich are tied at 3 percent each. Former Hewlett-Packard CEO Carly Fiorina rounds out the GOP field with 2 percent.

No other Republican candidate beside Trump, Cruz and Rubio have been in double-digits since the first survey was conducted about six weeks ago.

The online poll of 2,887 registered Republican voters nationwide was conducted Feb. 1-7 and carries a margin of error of plus or minus 2.7 percentage points.