Three influential leaders in the conservative movement have summoned other top conservatives for a closed-door meeting this Thursday in Washington D.C. to talk about how to stop Donald Trump and, should he become the Republican nominee, how to run a third-party “true conservative” challenger in the fall.

The organizers of the meeting include Bill Wichterman, who was President George W. Bush’s liaison to the conservative movement, Bob Fischer, a South Dakota businessman and longtime conservative convener, and Erick Erickson, the outspoken Trump opponent and conservative activist who founded RedState.com.

“Please join other conservative leaders to strategize how to defeat Donald Trump for the Republican nomination,” the three wrote in an invitation obtained by POLITICO that recently went out to conservative leaders, “and if he is the Republican nominee for president, to offer a true conservative candidate in the general election.”