Last week, during a White House press briefing on the coronavirus pandemic, President Trump answered a question from a reporter who suggested that all of the states should move toward mail-in (or “absentee”) voting in the upcoming primary and general elections.

The President pushed back strongly: “It shouldn’t be mail-in voting. … You don’t send it in the mail where people pick up—all sorts of bad thing can happen … by the time it gets in and is tabulated. No, it shouldn’t be mailed in. You should vote at the booth. And you should have voter ID, because when you have voter ID, that’s the real deal.”

President Trump was absolutely right to highlight the dangers of mail-in voting. Compared to in-person voting with photo ID, the opportunities for voter fraud are much greater. Three forms of fraud occur wherever mail-in voting is allowed.

1. Double voting. During my two terms as Kansas Secretary of State, I was the only secretary of state in the country with the power to prosecute election crimes. The most frequent form of voter fraud that my office prosecuted was double-voting—in which the voter votes in two or more states, usually through mail-in voting. It occurs with shocking frequency. Indeed, a 2017 study looking at just 21 states revealed that approximately 8,400 people had double-voted in the 2016 election.

2. Ballot harvesting. Mail-in voting also facilitates a second form of fraud: ballot harvesting—a form of fraud whereby someone purporting to deliver the completed ballots on behalf of voters only delivers those ballots believed to favor a particular candidate. The country was introduced to ballot harvesting on the national stage in 2018 when the Mark Harris v. Dan McCready race for North Carolina’s 9th Congressional District was in question for months, due to credible allegations that a contractor working for the Harris campaign had engaged in ballot harvesting.

As I wrote at the time, because the victim of the fraud was the Democrat, Nancy Pelosi suddenly changed her tune on the subject. She went from mocking allegations of voter fraud to describing just how significant the problem of voter fraud is: “This is bigger than that one seat. This is about undermining the integrity of our elections. … What was done there was so remarkable, in that that person, those entities, got away with that.” Now she seems to have forgotten her own words on the subject.

3. Falsely voting another person’s ballot. A third form of fraud through mail-in voting occurs when someone fraudulently requests another person’s mail-in ballot. The fraudster then intercepts the ballot at the real voter’s mailbox, or he has the ballot sent to a phony address. He can then fraudulently vote the ballot and send it in.

Alternatively, this form of fraud can be used without intercepting the ballot. The fraudster simply requests mail-in ballots for others to increase voter turnout in certain neighborhoods. Before I became Kansas Secretary of State, this form of fraud occurred more than 50 times in urban Wyandotte County, Kansas. It stopped when I introduced mail-in ballot security reforms to Kansas law. More on that below.

All three of these forms of fraud occur in the vast majority of states, which allow voters to choose to vote by mail. All except Kansas, that is, where two out of the three problems have been solved by the security reforms that I drafted and the Kansas Legislature enacted in 2011.

4. Massive fraud in all-mail elections. There is a fourth form of voter fraud that is widespread in the three states that have all-mail voting—Colorado, Oregon, and Washington state. Those states don’t give the voter a choice. Instead, in every election, every registered voter is automatically sent a ballot. The results have been disastrous.

Because the states’ voter rolls include so many invalid names—deceased voters, people who have moved away, noncitizens, and duplicate registrations—huge numbers of extra ballots are sent out by the relevant election officials to invalid names on the rolls. In 2012, the Pew Center on the States estimated that the voter rolls nationwide contain 24 million invalid names. As a result, nearly 250 counties have more names on the voter rolls than there are eligible voters living in the county.

As a result, hundreds of thousands of extra ballots are sent out. As the Colorado Secretary of State told me when Colorado first introduced all-mail balloting, it was a fiasco. Untold numbers of additional ballots arrived at addresses throughout the state. Even one of the Colorado state legislators received two ballots—she was double-registered under her maiden name and her married name.

When these additional ballots arrive in their mail box, many voters will be tempted to fill them out and send them in. It is a massive invitation to fraud.

Democrats in Congress, including Senator Elizabeth Warren (D-MA), have been calling for new federal law that would mandate all-mail balloting in all 50 states as a response to the coronavirus pandemic. Such a law would guarantee voter fraud on a scale never seen before; it should be opposed at all costs. Democrats in states such as Texas and Arizona have been calling for all-mail elections at the state level too.

Assuming this disaster can be avoided, a likely scenario in elections this year will be that state election officials encourage voters to request absentee ballots on their own, under existing state laws. That would be better than an all-mail election mandate coming from Congress, but it would still be vulnerable to first three forms of fraud described above.

The solution is for state legislatures to take steps now to secure mail-in ballots like we did in Kansas. We added three security elements to mail-in voting that operate to prevent voter fraud. First, we required virtual photo ID; the person requesting the absentee ballot must either send in a photocopy of a qualifying photo ID or write his complete driver’s license number on the ballot-request form. Second, we required signature verification. Every voter’s signature has to be matched against the signature that the state has on file for that person. If the signature doesn’t match, the voter is contacted and allowed to try signing again. If the signature still doesn’t match, no ballot is sent out. Third, in a separate law, Kansas also deterred ballot harvesting by requiring the voter of a delivered ballot to sign the envelope and authorize a specific, named person to deliver the ballot on his behalf.

It is essential that the other states enact these reforms to make mail-in ballots more secure. If all states were to take these three steps, the mail-in ballot fraud that President Trump rightly pointed out could be avoided. But the time to act is now.

Kris W. Kobach served as the Secretary of State of Kansas during 2011-2019. He drafted and implemented Kansas’s Photo-ID and Proof-of-Citizenship election law. His office also prosecuted more than a dozen cases of election fraud in Kansas. An expert in immigration law and policy, he coauthored the Arizona SB-1070 immigration law and represented in federal court the 10 ICE agents who sued to stop Obama’s 2012 DACA executive amnesty. He is currently a candidate for the U.S. Senate. His website is https://kriskobach.com/.