But dozens of states already have in place more accessible voting methods, Business Insider reported , with 38 allowing early voting, 35 allowing absentee ballots and five relying on a mail-in system to conduct their elections. Republicans are elected to every level of government in those states, including those that primarily vote by mail.

Several surveys indicate that Democrats receive more favorable outcomes as the voting process is simplified, the Seattle Times reported.

"Tremendous potential for voter fraud, and for whatever reason, doesn't work out well for Republicans."

"Republicans should fight very hard when it comes to state wide mail-in voting. Democrats are clamoring for it," Trump tweeted.

President Donald Trump took aim Wednesday at voting by mail, saying Republicans lose out in the process that several states are already using to cut the risk of transmitting the coronavirus.

Legal battles in Wisconsin, which held its primary Tuesday despite the threat of the pandemic, gave a taste of what could come in November's presidential election. The U.S. Supreme Court reversed a lower court order extending Wisconsin's deadline for filing absentee ballots, after the state's top court separately blocked Democratic Gov. Tony Evers' executive order to delay in-person voting.

Hours after Trump warned against mail-in voting, two key GOP officials pushed voters in their state to cast their ballots through the mail.

Ohio Gov. Mike DeWine and Secretary of State Frank LaRose, both Republicans, announced Wednesday that they're encouraging people to vote by mail, a direct contradiction to Trump's wishes. The state is crucial for both Trump and apparent Democratic nominee former Vice President Joe Biden. Former President Barack Obama won Ohio in 2008 and in 2012 during his reelection. Trump flipped Ohio in 2016.

Easier access to voting by mail could affect Trump's own fortunes in his bid for reelection this year, especially in swing states where margins could be razor thin.

This isn't the first time Trump criticized the vote-by-mail process. On "Fox and Friends" last week, Trump slammed Democrats' vote-by-mail proposals in Congress, saying, "They had a level of voting that, if you ever agreed to it, you'd never have a Republican elected in this country again."

MSNBC tweet

Those voter protection proposals were left out of the $2 trillion relief package Congress enacted to stem the economic damage from the pandemic.

Still, last week, House Speaker Nancy Pelosi continued to call for funding such measures. She predicted that America will need to move toward a "vote by mail" system to give citizens a safe way to elect their lawmakers while the coronavirus makes it dangerous to congregate.

"In terms of the elections, I think we'll probably be moving to vote by mail," Pelosi said in an interview on MSNBC. "That's why we wanted to have more resources in this third bill that just was signed by the president, to get those resources to the states to facilitate the reality of life: that we are going to have to have more vote by mail."

More than a dozen states and U.S. territories have already adjusted their own presidential primaries, with some opting for a mail-in system to replace in-person voting and others delaying the primaries entirely.

The adjustments have forced both Democratic presidential candidates, former Vice President Joe Biden and Sen. Bernie Sanders, to change the way they get their message out to voters. Both campaigns no longer do in-person events or rallies and have been relying on digital outreach to connect with voters.

The coronavirus has spread to dozens of countries globally, with more than 1.4 million confirmed cases worldwide and over 82,992 deaths so far, according to data from Johns Hopkins University. There are at least 399,900 cases in the United States and at least 12,911 deaths, according to the latest tallies.

— CNBC's Jacob Pramuk and Christina Wilkie contributed to this report