WASHINGTON — For months, Ted Cruz and Marco Rubio tried to tear each other down in a bid to become Donald Trump’s top challenger. The senators are now unlikely allies in an effort to stop Trump’s march toward the Republican nomination.

The strategy shift was on full display in Thursday’s GOP debate. Rubio and Cruz ignored each other almost entirely, choosing to instead fully focus their attention on the billionaire businessman whose surprising dominance of the 2016 race has shaken the Republican Party to its core.

Cruz and Rubio’s moves underscore the desperate situation in which they find themselves. A clear path to winning the nomination through the traditional primary process seems increasingly out of reach. Instead, they’re eying a deeply complicated, long-shot plan to stop Trump at a contested convention.

It’s a blueprint that probably only works if Rubio and Cruz, along with Ohio Gov. John Kasich, stay in the race, siphoning delegates away from Trump as the primary process weaves its way across the country for months to come.