Injecting further uncertainty is the question of when a new budget will be passed to get the government up and running again.
"The longer it goes, the more damaging it becomes because of the accumulation of unpaid bills and more people being affected by not being paid," Parker says.
Here's a rundown of who would likely suffer most from a government shutdown, and who would escape relatively unscathed.
Biggest losers in a government shutdown
Government contractors and vendors. Government spending accounts for roughly one-fifth of the U.S. gross domestic product, and much of that adds to the bottom lines of government contractors and vendors. A government shutdown likely would mean that payments to vendors and contractors would stop, Parker says.
International travelers. During the last government shutdown, the Department of State slowed the application process for new passports and visas, leading to long delays for those seeking to travel internationally.
Taxpayers expecting a refund. A government shutdown could delay the issuance of tax refund checks and the processing of tax returns, especially for those who sent in paper returns.
"The (Internal Revenue Service) would basically shut down, so there would be no one there to process your taxes or send you your refunds," Goldwein says. But you still need to pay your taxes on time.
Recent retirees. If the last government shutdown is any indication, new applications for Social Security benefits would be significantly slowed, resulting in lengthy delays for new beneficiaries.
"There will be no employees on the job to process the applications, which require more intense handling than just simply the mailing of checks, which is fairly automated," Parker says.
Government employees. Nearly all government workers would see their paychecks delayed by a government shutdown. Those deemed "essential," such as active military, corrections officers, firefighters and utilities, would continue working with reduced or delayed pay until things returned to normal. It is likely they'd be reimbursed their missing salary, Goldwein says. Nonessential workers would be furloughed, possibly without pay.
New Medicare and Medicaid patients. Applicants for Medicare and Medicaid benefits likely would face delays because there wouldn't be employees to process them.