President Trump's tweets don't pack the punch they did at the outset of his presidency. His Twitter interaction rate — a measure of the impact given how much he tweets and how many people follow him — has tumbled precipitously, according to data from CrowdTangle.

Why it matters: It's a sign that his strongest communication tool may be losing its effectiveness and that the novelty has worn off.

Trump's interaction rate has fallen from 0.55% in the month he was elected to 0.32% in June 2017 — and down to 0.16% this month through May 25. (The metric measures retweets and likes per tweet divided by the size of his following.)

Expand chart Data: CrowdTangle; Note: Interaction rate is retweets plus likes per tweet divided by total followers. Explore the tweets that got the most interactions by month here; Chart: Naema Ahmed/Axios

Trump's lines of attack have been repeated so much that they don't shock anymore, says Toronto Star Washington bureau chief Daniel Dale.

Because norm-breaking tweets have become the new norm, Dale says, he doesn't cover them as often — and therefore, casual readers hear about them less.

Attacking the Mueller investigation went from scandalous to routine for Trump, and accusing government officials of treason went from groundbreaking to commonplace.

Since April 1, Trump has tweeted about:

"No Collusion" 54 times

"No Obstruction" 30 times

"Witch Hunt" 20 times

"Hoax" 19 times

"Radical" Left/Democrats 17 times

13 or 18 "Angry Democrats" 12 times

"Presidential Harassment" 10 times

"Treason" 7 times

The big picture: While the number of interactions per tweet Trump generates has increased 21% between his first six months and most recent six months, it lags way behind his follower growth of 110%.

And he's tweeting more, which could make any individual tweet less likely to stand out.

The pace of Trump's tweeting has picked up over the course of his presidency, from 157 times per month during his first 6 months to 284 times per month over the last 6 months.

has picked up over the course of his presidency, from 157 times per month during his first 6 months to 284 times per month over the last 6 months. May is on pace to be the lowest month for Trump's Twitter interaction rate since January 2016.

to be the lowest month for Trump's Twitter interaction rate since January 2016. Not counting posts he retweets, he is at 343 tweets through May 25 — closing in on his one-month record of 348 in August.

By the numbers: