A new weekly national tracking poll from NBC News shows Donald Trump adding to his lead over the Republican field, while Hillary Clinton’s lead over Bernie Sanders narrows slightly.

Trump has the support of 38 percent of Republicans nationally, 20 points higher than his nearest rival Ted Cruz, with 18 percent. Hillary Clinton leads Sanders by 10 points, 50-40.

The poll was conducted last week, Monday through Sunday, so it mostly doesn’t reflect changes from Saturday’s GOP Presidential debate. According to the survey, however, Trump gained three points over the week, while Cruz dropped two points. Marco Rubio also lost ground, dropping three points, while John Kasich rose four points and Jeb Bush gained a single point.

The overall breakdown on the Republican field is Trump (38 percent), Cruz (18), Rubio (14), Carson (8), Kasich (7), and Jeb Bush (4).

Trump’s 38 percent support is the highest its been since right before the Iowa caucus. The number of Republicans who think Trump will ultimately win the nomination is also rising. Before the Iowa caucus, 62 percent of Republicans said he would be the nominee. In the week after the Iowa caucus, that number dipped down to 42 percent. This week, 56 percent of Republicans think Trump will be the nominee.

Trump’s lead has also rebounded. Last week, which covered the period after the Iowa caucus but before the New Hampshire primary, Trump led the GOP field by 15 points. This week, after his landslide win in New Hampshire, his lead is back up to 20 points.

It is important to remember that, as a national tracking poll, the NBC News/Survey Monkey survey is a lagging indicator of the state of the campaign in many respects. The NBC/Survey Monkey tracking poll released on the day of the Iowa caucus, for example, showed Trump with a 19 point national lead.

The tracking poll released on the eve of Trump’s win in New Hampshire showed the Republican race tightening nationally ahead of the vote. The national polls don’t really reflect the possible outcome in state primaries, although the state contests will eventually impact the national polls.

In the Democrats case, the national tracking poll simply reflects what has become clear since voting started in that contest. Hillary Clinton is increasingly in trouble in her race against Bernie Sanders.

Her 10-point lead over Sanders is her smallest lead ever in the NBC tracking poll. In addition, all other public polling shows a tightening of her race against Sanders. Democrats are voting this weekend in Nevada, and recent polling suggests Sanders will do better than expected.

The following weekend, Democrats in South Carolina will vote in that primary. Although Clinton still maintains a wide lead in that primary, her margin has been falling. If Sanders performs better than expected in either or both of these contests, the national numbers likely will tighten even more.

There is a certain amount of conventional wisdom which cautions that early contests aren’t as meaningful because relatively few delegates are at stake. While that is true, the early contests can have a dramatic impact on national polls. These polls can then drive expectations for the later contests, when considerably more delegates are at stake.

This is the negative feedback loop that threatens to swamp Hillary Clinton’s campaign. She needs big wins in Nevada this weekend and South Carolina next week to reclaim her national narrative.