The latest Fox News poll shows Donald Trump opening an 18-point lead nationwide over second place Texas Sen. Ted Cruz. It’s a big jump from three weeks ago, when Fox News showed Trump with a narrow 3-point lead over Cruz, 41-38.

Trump’s gain, according to the poll, is due more to a collapse of support for Cruz rather than a jump in support for the leader.

In this survey, Trump has the backing of 45 percent of Republicans nationwide. Three weeks ago, Trump’s support was 41 percent, a change that is within the poll’s margin of error. Cruz’s support, however, has apparently fallen from 38 percent in late March to just 27 percent today.

Support for John Kasich, meanwhile, has surged 8 points, rising from 17 to 25 percent support. Trump leads within every demographic group, while Cruz and Kasich are tied in all except evangelicals and “very conservative” voters.

Trump’s lead in this Fox poll is the largest he’s recorded in the poll since the middle of December, when he led a much more crowded field by 21 points. Interestingly, he had gained just 6 points of support since then, rising from 39 to 45 percent support. The fluidity in the campaign has been the result of voters moving from one challenger to another.

While Trump’s position in the Republican primary has improved in the Fox poll, his general election prospects have not. He trails Hillary Clinton by 7 points in a hypothetical match-up, with 20 percent of Republicans supporting Clinton. Cruz and Clinton are tied and Kasich beats her by 9 points.

Trump trails Vermont socialist Sen. Bernie Sanders by 14 points. Cruz trails Sanders by 12 and Kasich trails by 4 points.

Most recent public polls have shown a tightening of the race between Trump and Cruz. The most recent poll for CBS News shows Trump with a 13 point lead, down from 20. IBD/TIPP went from Trump +11 to Trump +7 in their most recent survey. Reuters tracking has also shown a drop in Trump’s lead.

Although it is probably too soon to show up in polling, it is possible that some of the drop in Cruz’s support is fallout from controversy over the delegate voting process in Colorado. The Fox poll was conducted as that controversy was intensifying in the media, but there is generally a lag time before big events show up in polling.

It is also possible that the Fox poll is simply an outlier, since it is based on a large move between Cruz and Kasich, rather than a surge of support for Trump. It is also possible that, as the race hits the final stretch, voters who oppose Trump are divided on which alternative candidate to support.

It is possible that Republicans will enter their convention in Cleveland where no candidate, including their frontrunner, commands a majority of support from their voters across the country. Conventions are designed to unify a party ahead of a general election. This year’s convention may test that challenge like never before.

Mike Flynn is a delegate for Ted Cruz in Illinois Congressional District 18.