Hatch, 83, said Tuesday he's always been a fighter, "but every good fighter knows when to hang up the gloves".
If he ran, Romney would enter the Senate race as the heavy favorite, having carried Utah in 2012 by a margin of almost 3-to-1 over Democrat Barack Obama.
Hatch's decision not to pursue re-election is bound to shake up Utah state politics, and it is also likely to generate intrigue over the rumors that Mitt Romney is thinking about running to take his place.
Hatch, 83, has served in the Senate since 1977 and is now the longest serving Republican in the Senate.
That is in part because Mr. Hatch had privately told Mr. Romney he was not sure he was ready to leave a seat he has held since 1977 and White House officials did all they could to nudge him into another campaign.
Hatchs decision comes just weeks after Trump signed a sweeping tax overhaul into law, a measure that the senator helped write as chairman of the Finance Committee. "I will never forget the (beyond kind) statements he has made about me as President", he added. Asked if he thought Romney would be interested in coming to Washington during the Trump era, Trump replied, "I don't know".
In a video announcement he posted online, Hatch reflected on his time as an amateur boxer in his youth as he explained his desire to step down early next year.
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Hatch, 83, is the longest-serving GOP Senator in USA history, having first been elected in 1977.
After the election, Romney submitted himself as a candidate to be secretary of state in an excruciatingly public interview process.
That dynamic sparked a recent lobbying effort by the White House to encourage Hatch to seek another term in office.
In the statement announcing his decision not to run again, Hatch cited work with the disadvantaged among his accomplishments, including helping create the Americans with Disabilities Act, expanding children's health insurance and expanding use of generic drugs.
In 2000, Hatch sought the Republican nomination for president, saying he had more experience in Washington than his opponents and insisting he could work with Democrats.
"The Utah Republican Party is quite conservative", said Phill Wright, a member of the state Republican central committee and a former party vice chairman. He withdrew after only winning 1 percent of the vote in the Iowa caucuses.
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