Singer’s impending decision comes at a potentially pivotal time for former Florida governor Jeb Bush, who is struggling with deteriorating poll numbers and turned in a weak debate performance on Wednesday evening in Colorado. Singer’s move will raise further questions about the effectiveness of the Bush campaign’s efforts to staunch the flow of money and support to Rubio with attacks on his Senate record and whispers about misdeeds in his past. 

Given the timing of Singer’s move, speculation that Bush’s performance in Wednesday’s debate pushed the billionaire hedge-fund magnate into Rubio’s camp will be unavoidable. Bush, who has long promised to run a joyful and optimistic campaign, made headlines in the debate by attacking his one-time protégé for missing Senate votes…

Above and beyond the seven-figure checks he’s capable of cutting to candidates and political-action committees, his endorsement carries sway in the wider community of wealthy Republican donors. He has emerged as one of the biggest bundlers of hard money in the Republican party, raising hundreds of thousands of dollars for Senate candidates in the 2014 election cycle. Over the past several years, he’s united donors who favor an aggressive, internationalist foreign policy — including Citadel’s Kenneth Griffin and AQR Capital’s Cliff Asness — to build a broad fundraising network ​that many say is second only to the libertarian Koch brothers’ in its strength. Singer is also a supporter of conservative think tanks like the American Enterprise Institute and the Manhattan Institute, activist organizations like the Federalist Society, and super PACs like the Club for Growth and Ending Spending.