Democrats may have a big problem in Michigan this November — and a legal fight in the shorter term. A decision invalidating petition signatures for John Conyers will keep his name off of the primary ballot, but Conyers and his team vow to win … in the courts, anyway:

Rep. John Conyers (D-Mich.) won’t appear on the Democratic primary ballot after failing to submit enough valid signatures, Wayne County Clerk Cathy Garrett said on Tuesday. …

Conyers’s Democratic primary opponent, pastor Horace Sheffield III, challenged the validity of the incumbent’s signatures. The clerk ruled that since two of his petition-gatherers weren’t registered voters in Michigan, as required under state law, the signatures he obtained didn’t count.

Conyers submitted 2,000 signatures, needing 1,000 valid ones. After the challenges, he had 592.

Legal challenges to the petition-gathering rule are likely, though. The American Civil Liberties Union has already challenged that law in federal court, saying it’s unconstitutional to require signature-gatherers to be registered voters.

Jazz raised this possibility almost two weeks ago, and Mary Katharine noted it in passing last night. It seemed fantastical that a man who won 25 terms in office could get tripped up on such a basic issue, but here we are. Apparently, Conyers’ expert staff couldn’t be bothered to check the CVs of their petition gatherers despite state law requiring it, which tells us plenty about the quality of people Conyers has in his employ.

Meanwhile, his lawyers plan to challenge the ruling, but it’s not clear on what basis they could succeed:

The 84-year-old Detroit Democrat was at risk because officials believe several people who circulated his petitions do not appear to have been registered voters or had registered too late.

Conyers has three days to appeal Garrett’s ruling to the Michigan Secretary of State.

“It is a very unfortunate circumstance that an issue with a circulator of a petition would disqualify the signature of valid registered voter,” Garrett said. “However, I am bound by the current laws and statutes of the state of Michigan that set forth very specific and narrow instructions regarding candidate petitions.”

Federal court actions, meanwhile, are taking aim at the requirement that petition collectors be registered voters. The ACLU has filed suit to change state law. If neither happens, Conyers can run as a write-in. If he wins the primary as a write-in, he would qualify for the general election ballot.

The ACLU can file all the suits it wants to change the law, but that takes a legislature if the law itself isn’t unconstitutional or overwhelmingly unreasonable. Conyers managed to get elected to the same office 25 times while abiding by the law, or at least avoiding challenges that alleged he didn’t. One round of incompetence doesn’t provide a good reason for a court ruling that allows Conyers onto a ballot when his competition abided by the same law.

They’d better start planning their write-in campaign, and hope that Conyers’ name recognition can carry the day. If he manages to win the primary, his name will automatically appear on the general-election ballot. But maybe voters in MI-13 should consider whether this episode isn’t a signal that their 50-year Congressman has lost his grip, and start looking for a fresh face.

Note: Bear in mind that for all intents and purposes, this is the general election. MI-13 has a D+34 Cook rating, so there is zero chance for a Republican surprise in the fall.