As Sen. Mitt Romney, R-Utah, voted Wednesday to convict President Trump on one of the two articles of impeachment against him – abuse of power – Trump's son Donald Trump Jr. was already calling for Romney's expulsion from the Republican Party.

"Mitt Romney is forever bitter that he will never be POTUS," Trump Jr. tweeted after Romney announced his decision. "He was too weak to beat the Democrats so he's joining them now. He's officially a member of the resistance & should be expelled from the GOP."

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Trump Jr. then started tweeting the hashtag "#ExpelMitt" and later tweeted that Romney "should be expelled from the @SenateGOP conference."

Romney's fellow Republican senators, while making clear they disagreed with his vote, seemed to stand by him despite the attacks from Trump Jr.

"Nope, not at all," Sen. Lisa Murkowski, R-Alaska, said in response to a question asking if Romney should be expelled from the party.

"I think Mitt’s decision was a mistake," Sen. Ted Cruz, R-Texas, told reporters, before adding that he had no hard feelings toward Romney.

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"I disagree with him on it. At the end of the day every senator has to make his or her decision as to how we will comply with the obligations [of the] constitution."

When Sen. Kevin Cramer, R-N.D., was asked by reporters whether he agreed with Trump Jr. that Romney should be expelled, he answered that he did not know how such a move was even possible.

"I don’t really know how you expel someone from the Republican Party," he said, according to Talking Points Memo. "I’m glad Mitt’s a Republican."

So can a senator be expelled from the Republican Conference for a vote on a certain issue? Or the national party? Or a state party?

The U.S. political system may be run by two major parties, but compared with other countries, those parties have very little control over the positions espoused and votes cast by their members.

The Republican National Committee has nothing in its rules about expelling an individual member from the party. In fact, while it is active as a campaign arm of the GOP and provides guidance to state parties and individual members on the general direction of the party, the RNC typically only has full control over the national party convention it hosts every four years. Most of its rules have to do with filling various party offices, delegate selection for the national convention and the procedure by which its convention runs.

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The only rule the RNC has regarding support for individual candidates bans it from supporting candidates in contested primaries and stipulates that someone running for another party's nomination cannot be recognized as the GOP nominee.

The Utah Republican Party does have a provision for expelling candidates from party membership, but it would not apply to Romney. A rule in the Utah GOP's by-laws mandates that candidates running for office as a Republican have to sign an agreement saying they support the state party's platform with a list of any platform stances they disagree with.

Candidates who do not sign the agreement "will automatically forfeit their Party membership two days following the State designated candidate filing-period deadline."

The Utah GOP platform says nothing about Trump or impeachment.

Trump Jr. did specifically say, however, that Romney should be expelled from the Senate GOP conference. Are there rules allowing for the expulsion of members in its by-laws?

No. The Senate Republican Conference rules explicitly say that it does not control how its members vote.

"The Republican Conference has never been a caucus in the dictionary sense, that is, a 'partisan legislative group that uses caucus procedures to make decisions binding on its members," its rules say. "Even during the tense years of Reconstruction, Republican Senators were not bound to vote according to Conference decisions."

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A 1925 resolution in the Senate GOP conference reads: "Be It Resolved: That no Senator attending this Conference or any Conference held hereafter shall be deemed to be bound in any way by any action taken by such Conference, but he shall be entirely free to act upon any matter considered by the Conference as his judgment may dictate, and it shall not be necessary for any Senator to give notice of his intention to take action different from any recommended by the Conference."

It is technically possible that the Senate GOP could amend its rules by a majority vote to include a procedure for expelling members, then subsequently expel Romney. The House Republican Conference has a rule allowing for the expulsion of its members with a two-thirds vote.

But as the rules stand right now, Romney cannot be expelled from the national or state party or Senate GOP conference. That is also very unlikely to change considering the support Romney has received from at least some of his Senate GOP colleagues.

Fox News' Jason Donner contributed to this report.