Three senior groups backed President Biden’s reelection bid Wednesday based on his pledge to protect Social Security, but so far he has only offered vague promises to protect the program as insolvency looms. The National Committee to Preserve Social Security and Medicare, which bucked its 38-year tradition of avoiding endorsements to back Mr. Biden in 2020, has done so again this election. Social Security Works PAC and the bipartisan National United Committee to Protect Pensions also said...