DES MOINES — Four Democrats running for president pledged Monday to keep Social Security solvent for senior citizens by raising the income cap so wealthier Americans pay more taxes to support the system without changing eligibility requirements.The 2020 hopefuls who participated in a forum sponsored by AARP Iowa and the Des Moines Register also supported adding a public option to the Affordable Care Act to expand coverage to more Americans but three balked at eliminating private insurance for those who prefer to keep their current health care coverage.Former Vice President Joe Biden, New Jersey U.S.