You can file a lawsuit over anything.

A judge might throw your case out rather than let it go to court if it doesn't even warrant a case, but something being "litigable" isn't much of a bar.

If someone repeatedly files frivolous lawsuits purely for the purpose of harassing someone, they might be guilty of barratry, depending upon locale.

https://en.wikipedia.org/wiki/Barratry_%28common_law%29

Barratry (/ˈbærətri/ BARR-ə-tree, from Old French barat ("deceit, trickery")) is a legal term that, at common law, described a criminal offense committed by people who are overly officious in instigating or encouraging prosecution of groundless litigation, or who bring repeated or persistent acts of litigation for the purposes of profit or harassment.

Although it remains a crime in some jurisdictions, barratry has frequently been abolished as being anachronistic and obsolete.

If barratrous litigation is deemed to be for the purpose of silencing critics, it is known as a strategic lawsuit against public participation (SLAPP). Jurisdictions that otherwise have no barratry laws may have SLAPP laws.

United States

Several jurisdictions in the United States have declared barratry (in the sense of a frivolous or harassing litigant) to be a crime as part of their tort reform efforts. For example, in the U.S. states of California, Oklahoma, Pennsylvania, Virginia, and Washington, barratry is a misdemeanor. In Texas, barratry is a misdemeanor on the first conviction, but a felony on subsequent convictions.

• California Penal Code Section 158: "Common barratry is the practice of exciting groundless judicial proceedings, and is punishable by imprisonment in the county jail not exceeding six months and by fine not exceeding one thousand dollars ($1,000)."

• California Penal Code Section 159: "No person can be convicted of common barratry except upon proof that he has excited suits or proceedings at law in at least three instances, and with a corrupt or malicious intent to vex and annoy."

• Revised Code of Washington 9.12.010: "Every person who brings on his or her own behalf, or instigates, incites, or encourages another to bring, any false suit at law or in equity in any court of this state, with intent thereby to distress or harass a defendant in the suit, or who serves or sends any paper or document purporting to be or resembling a judicial process, that is not in fact a judicial process, is guilty of a misdemeanor; and in case the person offending is an attorney, he or she may, in addition thereto be disbarred from practicing law within this state."

• Virginia laws on barratry, champerty, and maintenance were overturned by the Supreme Court of the United States in NAACP v. Button 371 U.S. 415 (1963).

• Vermont Statutes Title 13, § 701: "A person who is a common barrator shall be fined not more than $50.00 and become bound with sufficient surety for his or her good behavior for not less than one year."

Ha! According to the supreme court, Biden could explain all of this to Johnson and the rest of the republicans using guns.

It's not a federal legal matter, party internal politics are not regulated the same way as actual elections. There's some tangential finance laws, but the democratic party can pick whoever the hell they want however the hell they want.