WATCH: GOP senator Orrin Hatch to retire -- opening the door to Trump critics like Romney

Senator Orrin Hatch (R-UT) announced via Twitter that he would not be running for reelection in the 2018 midterms.

The announcement came just under 40 years since the President pro tempore was first sworn in to the United States Senate on January 3, 1977.

Former Governor Mitt Romney (R-MA) is considered the GOP frontrunner to succeed Sen. Hatch, despite President Donald Trump's open contempt for his GOP presidential nominee predecessor.

In December, President Donald Trump attempted to block Romney's candidacy by convincing Hatch to run for an eighth term in the U.S. Senate.

President Trump's effort was apparently unsuccessful in closing the deal with Sen. Hatch.