Rep. Jason Altmire (D-Pa.) may be barred from running in the Democratic primary against fellow Rep. Mark Critz (D-Pa.), a move that would make it all but impossible for him to return to Congress next fall.

A judge will rule Monday whether Altmire submitted enough valid signatures in his petition to get on the ballot, and he is dangerously close to falling below the needed number.

The two were thrown into the same congressional district in redistricting, and their primary has already been testy. Altmire has represented more of the new district, but many unions are furious with him for his health care vote and have rallied around Critz. The primary is expected to be a dog-fight — if it happens.

Pennsylvania law requires 1,000 valid signatures for a candidate to get on the ballot. Altmire submitted more than 1,600 but Critz's campaign challenged the validity of more than half of those, and the judge has agreed with them on many points. One ruling is left and Altmire is down to approximately 1,150 signatures — if this ruling goes against him he'll be stripped from the ballot.

The issue at hand: Pennsylvania law requires petition circulators to live within the congressional district, and Altmire's staffer who gathered the signatures splits her time between her parents' home, in the district, and a Pittsburgh apartment outside of it.

A Critz campaign staffer said whether or not Altmire will prevail is "basically a coin flip" at this point.

If the judge decides her primary residence is in Pittsburgh, Altmire will not be able to appear on the ballot, a body blow to his campaign and a huge boon for Critz.

Altmire's campaign did not immediately respond to a request for comment.

Even if Altmire survives the challenge, it's a bad sign for his campaign: Seasoned politicians almost never fail to gather enough ballot petition signatures, and Critz's team submitted double the number of signatures that Altmire did for their campaign.

This scenario is unusual but not unprecedented: Former Rep. Charlie Wilson (D-Ohio) failed to make the ballot in his first run for Congress in 2006 and had to win a write-in primary election. This year, many Republican presidential candidates failed to collect the 10,000 signatures necessary to get onto Virginia's primary ballot.