(NEXSTAR) – Months ahead of the midterm elections, Congress could pass legislation that would enact new, strict identification requirements that could impact how you vote. The bill, called the Safeguard American Voter Eligibility (or SAVE Act or SAVE America Act), would require Americans to prove they are citizens when they register to vote, mostly through a valid U.S. passport or birth certificate. It would also require a valid photo identification before voters can cast ballots, which some states already demand. The House passed the bill last week, sending it to the Senate, where it could face a tough fight. If it doesn’t pass, President Trump has said he would issue an executive order to enact stricter voter ID rules. Republicans said the legislation is needed to prevent voter fraud, but Democrats warn it will disenfranchise millions of Americans by making it harder to vote. Federal law already requires that voters in national elections be U.S. citizens, but there’s no requirement to provide documentary proof. Experts said voter fraud is extremely rare, and very few noncitizens ever slip through the cracks. Fewer than one in 10 Americans don't have paperwork proving they are citizens. Many states already require voters to show an ID to vote, but proving citizenship is more difficult. Here’s what to know. States primarily require a driver’s license or state ID and a Social Security number (or both) to register to vote. The new legislation would require additional documentation, like a passport or birth certificate, for a voter to become registered. REAL IDs would not be enough to prove citizenship, according to the Bipartisan Policy Center. The legislation also requires states to share their voter information with the Department of Homeland Security, as a way to verify the citizenship of the names on the voter rolls. That has drawn pushback from elections officials as potentially intrusive on people's privacy. Only 12 states and the District of Columbia will, with few exceptions, let you vote at the polls without any documentation under certain circumstances. That includes California, Illinois, Maine, Maryland, Massachusetts, Minnesota, Nevada, New Jersey, New Mexico, New York, Pennsylvania, and Vermont. These states require voters to submit at least one form of identification when registering to vote, and note that first-time voters will need to provide identification to polling workers. What qualifies as identification can vary by state. In California, for example, photo identification cards, student ID cards, a passport, a recent utility bill, or a document you received from a government agency are among the list of acceptable means of proving your identity. In the remaining 38 states, voters need to provide some form of ID at the polls. In most cases, a photo ID is required. That includes Alabama, Arkansas, Florida (which also requires a signature on the ID), Georgia, Idaho, Indiana, Iowa, Kansas, Kentucky, Louisiana, Mississippi, Missouri, Nebraska, New Hampshire, North Carolina, Ohio, Oklahoma, Rhode Island, South Carolina, South Dakota, Tennessee, Wisconsin, While Michigan says all voters with a photo ID must present it at the polls, voters without a photo ID or those who forgot to bring theirs can still vote if they sign an affidavit. Texas, Idaho, Louisiana, and South Dakota will also allow voters to sign affidavits to prove their eligibility. If a voter in Iowa forgot their ID, another registered voter can attest for them. Both will have to sign an oath. Many states also allow for provisional ballots to be cast. Alaska, Colorado, Connecticut, Delaware, Virginia, West Virginia, and Wyoming will accept IDs that do not have a photo. Arizona will accept either one photo ID, two forms of identification with a current address, or a photo ID and a form with a current address on it. Montana will accept a photo ID or other forms of identification that list a voter's name and current address. States that primarily vote by mail have slightly different requirements. In Hawaii, all registered voters automatically receive a ballot in the mail. To register, they need to provide a driver’s license or state ID and Social Security number. Oregon voters do not require a photo to register to vote while Utah does, or two forms of alternative IDs. While Washington is also primarily a vote-by-mail state, those who want to vote in person need to provide identification or sign a ballot declaration. The new rules in the bill would take effect immediately, if the bill is passed by both chambers of Congress and signed into law. But with primary elections getting underway next month, critics said the sudden shift would be difficult for state election officials to implement and potentially confuse voters. Voting experts have warned that more than 20 million U.S. citizens of voting age do not have proof of their citizenship readily available. Almost half of Americans do not have a U.S. passport. Karen Brinson Bell of Advance Elections, a nonpartisan consulting firm, said the bill adds numerous requirements for state and local election officials with no additional funding. “Election officials have a simple request of Congress — that you help share their burdens not add to them,” she said. The Associated Press contributed to this report.