WASHINGTON – Sen. Mitt Romney, the sole Republican who voted to convict President Donald Trump in his impeachment trial, was the only GOP senator not invited to serve on the president's new panel to advise on reopening the country during the coronavirus pandemic.

Of the 53 Senate Republicans, 52 now serve in the "Opening Up America Again Congressional Group," which even includes a dozen Senate Democrats – but not Romney.

The Utah senator, who was the Republican party's nominee for president in 2012, has been a frequent critic of Trump and, in turn, has felt the sting of Trump's attacks – which included the president calling him a "pompous a**" in a tweet.

Romney also was not welcomed on a Thursday phone call in which the president spoke with senators of both parties about the coronavirus and the potential for reopening the country for business, according to a senior administration official.

Asked about why Romney was left out, the official simply said, "he wasn't invited."

Mitt Romney::A solitary GOP voice battling Trump for the soul of the Republican Party

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The panel, which was rolled out Thursday, includes 65 senators and 32 House members. It aims to offer guidance and ideas for ways to safely reopen the economy once the coronavirus slows its spread across the country. On calls with members of the group Thursday, Trump heard questions about increased testing for COVID-19 and options for how the country could safely reopen without risking a resurgence in cases.

While Republicans make up the majority of the group, it also includes 12 Democratic senators, one independent and 10 House Democrats.

Trump and Romney have a complicated history and relationship. In 2016, Romney held a news conference to denounce the future president as a "phony" and a "fraud." But after Trump was elected, they were photographed at dinner together discussing Romney potentially serving as secretary of state.

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Since coming to the Senate in 2018, Romney has taken on the role of Trump's chief Republican critic. He has voiced opposition and concerns on many of the issues his colleagues typically attempt to shy away from discussing.

In an interview with USA TODAY in October, Romney said he's had a long career and is no longer at the point of worrying about the political ramifications of his actions.

"I'm not going to worry so much about the political consequence, or praise or lack thereof," he said when asked about his legacy. "And I hope that my kids down the road will say 'Yeah, you know our dad, our grandfather our great-grandfather was a person of integrity and honored his oath of office.'"

Mitt Romney:A solitary GOP voice battling Trump for the soul of the Republican Party

In February, Romney joined Senate Democrats in voting to convict Trump on an article accusing the president of abusing his power by trying to pressure Ukraine into announcing investigations of political rivals, including former Vice President Joe Biden, in exchange for releasing nearly $400 million in military aid. Romney voted against the second article charging Trump with obstruction of Congress. Trump was acquitted on both articles.

Contributing: David Jackson and Ledge King