President Donald Trump took to Twitter early Wednesday to defend the Second Amendment just a day after retired Supreme Court Justice John Paul Stevens called for its repeal.

Stevens, 97, wrote in an essay on the New York Times website that a repeal would weaken the National Rifle Association’s ability to “block constructive gun control legislation.”

Trump, who has in the past appeared to change positions on tougher gun laws, took a firm stance in the tweet, and issued a warning if Republicans come up short at the polls.

“THE SECOND AMENDMENT WILL NEVER BE REPEALED!” he tweeted. “As much as Democrats would like to see this happen, and despite the words yesterday of former Supreme Court Justice Stevens, NO WAY. We need more Republicans in 2018 and must ALWAYS hold the Supreme Court!”

Repealing the amendment would be extremely difficult.

An amendment to the Constitution can be proposed only by Congress with a two-thirds vote in both houses, or by a constitutional convention called for by two-thirds of the state legislatures. The amendment then has to be approved by three-quarters of the states.

Stevens was on the losing end of a 2008 ruling in which the high court held that the Second Amendment gives individuals the right to own a gun for self-defense. He had previously called for changing the Second Amendment to permit gun control.

Stevens said the decision in that case, District of Columbia v. Heller, “has provided the N.R.A. with a propaganda weapon of immense power.” Stevens retired from the court in 2010, after more than 35 years.

In his essay published Tuesday, Stevens talked about the “March for Our Lives” events on Saturday that drew crowds in cities across the country. Stevens said the demonstrations “reveal the broad public support for legislation to minimize the risk of mass killings of schoolchildren and others in our society.”

He said the support “is a clear sign to lawmakers to enact legislation prohibiting civilian ownership of semiautomatic weapons, increasing the minimum age to buy a gun from 18 to 21 years old, and establishing more comprehensive background checks on all purchasers of firearms.”

But Stevens called on demonstrators to “seek more effective and more lasting reform.”

“They should demand a repeal of the Second Amendment,” he wrote.

Other demonstrators Saturday, in Phoenix and Salt Lake City, called for gun ownership rights to continue being protected by the Second Amendment.

The Associated Press contributed to this report.