For the first time in the Republican primary, Reuters’ tracking poll shows Texas Sen. Ted Cruz leading Donald Trump among likely voters in the Republican primaries.

This is also the first time Donald Trump hasn’t led the tracking poll in 2016.

Cruz has the support of 39 percent of likely Republican voters nationwide, just ahead of Donald Trump, who has 37 percent support. Ohio Governor John Kasich is a distant third, with 23 percent support.

In the broader survey of all Republicans, rather than likely voters, Trump still has a four-point lead. This, however, is sharply down from the 28-point lead he enjoyed in the middle of March.

Trump’s current support among Republican likely voters is the lowest since the end of February, when Jeb Bush, Marco Rubio and Ben Carson were still in the race. Since the race narrowed to three remaining candidates, Trump has enjoyed the support of almost half of Republican voters, or 45 percent.

Trump’s collapse in the Reuters tracking poll began on March 26, when the GOP frontrunner topped out at 48 percent support. Since then, Trump has lost 13 points of support. He gained two points back in today’s tracking poll.

The Reuters tracking poll is based on a five-day rolling average of results. Each day, a new round of daily interviews are added to the tracking poll, while older interviews are dropped from the tracking poll. As a result, there can be daily blips in the poll. The overall trend, however, is unmistakable. Trump has lost a considerable amount of support in the past two weeks and Cruz’s support has surged.

The drop in Trump’s support came just days after he retweeted an unflattering photograph of Cruz’s wife, Heidi. In the following days, the campaigns engaged in a war of words over the candidates’ spouses. Trump also angered many on both sides of the abortion debate when he said in an interview that women who get abortions should be legally punished. The Trump campaign later clarified his remarks.

The drop in support for Trump comes at a critical time in his campaign. While he leads the overall race for delegates, there are growing questions over whether he will be able to secure the minimum 1,237 delegates necessary to win the nomination on the first ballot.

With an expected loss in the Wisconsin primary on April 5, Trump already faced a narrow road to avoid a brokered convention. If his support in national polls doesn’t rebound, that road will become almost impassible.

Up to this point, Trump’s campaign has been predicated, largely, on his lead in the polls. His drop in the Reuters tracking, which he has led throughout the campaign, may force his campaign to alter its message.

The few states remaining to vote are critical to Trump’s hopes of securing the nomination before the convention. How he responds to a possible drop in support, and the reasons behind that drop, may allow him to reboot his campaign. Or, it could foreshadow a long, hot summer in Cleveland this July.