The poll, commissioned by ABC News and the Washington Post, shows both senators beating Trump in a hypothetical two-way race. Cruz leads Trump, 51 percent to 41 percent; Rubio's lead is 51 percent to Trump's 45 percent.
Trump still has a plurality of support when all four remaining candidates are given as options to GOP primary voters. Trump garnered 34 percent support to 25 percent for Cruz, 18 percent for Rubio and 13 percent for Ohio Gov. John Kasich.
For Republicans hoping to block Trump's march toward the nomination, the poll offers renewed hope it is possible. Of course, the poll posits a hypothetical two-way race that is unlikely to happen at least until after March 15, when Rubio and Kasich wage last-ditch efforts to defeat Trump in winner-take-all primaries in their home states.
Support for the anyone-but-Trump movement is strongest in two key GOP voting blocs -- white evangelicals and self-described very conservative voters. Cruz dominates Trump one-on-one in those categories. Rubio wins among those voters as well, though by much smaller margins.
Of the respondents who said they do not want Trump to win the nomination, 63 percent said they would support a brokered GOP convention to deny him the party nod.
The poll surveyed 1,000 people. The portion specific to the Republican primary has a margin of error of 5 percentage points.