The NBC News |SurveyMonkey national weekly election tracking survey of the Democratic nomination contest shows a rapid narrowing of Hilary Clinton’s lead over Bernie Sanders among Latino voters over the past two weeks.

While our national surveys have shown little discernible trend among all Democrats since the Iowa Caucuses, the movement among Latino voters suggests that a critical part of the so-called firewall of support that Clinton’s campaign had hoped to rely on among non-white Democrats may be crumbling.

Latino voters will play a critical role in Saturday’s Nevada Caucuses. Hispanics make up 17 percent of the population of eligible voters in Nevada and were 15 percent of Democratic caucus-goers in 2008.

Among all Democrats, Sanders has gained significantly over the past four months, cutting a Clinton lead that had been hovering at or near 20 percentage points among all Democrats in November to just 11 percentage points (50 to 39 percent) on the eve of the Iowa Caucuses. That trend has leveled off over the past three weeks, however, with only minor fluctuation and no significant change in the overall national trend.

NBC News

Yet among Hispanic voters, what had been a two-to-one or better Clinton lead for the past six months has narrowed dramatically in the two weeks following the Iowa Caucuses. Where our surveys had shown Clinton leading by 20 percentages points or better during December and January, her advantage had reduced to just three percentage points (46 to 43 percent) on our most recent week of tracking (from February 8-February 14).

The Clinton firewall among African American Democrats, on the other hand, remains intact. In the past week, she led Sanders among black Democrats nationwide by a whopping 52 percentage points (70 to 18 percent), roughly comparable to the margins Clinton has enjoyed so far in 2016.