I just spoke with Massachusetts state Sen. Bruce Tarr, the assistant Republican leader, and he told me that if the bill to provide for a temporary appointment to Ted Kennedy's Senate seat passes tomorrow, there could be plenty of room for legal challenges -- and a lot is riding on the final enactment votes tomorrow.

I asked Tarr whether Massachusetts Republicans would themselves file lawsuits, and he did not specifically say as much. However, he laid out scenarios under which other citizens of Massachusetts could potentially bring legal action.

One big issue, Tarr explained, is that this bill has been designated as an emergency law, in order to have it take effect immediately. Normally, under the state constitution, laws can only take effect after 90 days, which would defeat the purpose of the immediate appointment. However, an emergency law also requires a two-thirds vote within the legislature in order to pass -- and in the votes that have been held so far, it was only 97-58 in the House, and 24-16 in the Senate.

It's possible that Gov. Deval Patrick may try to invoke a section of the constitution that enables the governor to declare an emergency and enact the law anyway, without necessarily having the two-thirds majorities. But Tarr argues that this only applies to bills that are being subjected to a referendum, which is not being done for this act. If the legislature doesn't end up voting by two thirds, and Patrick were to try to act in such a manner, Tarr said there could be an opening for a lawsuit. "There are a lot of unanswered questions," said Tarr. "But what there is, is a will to take action if the governor were to violate the law with a letter declaring that this is an emergency."

And if the Dems were to get the two-thirds majorities, the Dems might still not be out of the woods. I asked Tarr about the argument I'd previously been given from Minority Leader Richard Tisei -- that the law is potentially unconstitutional on its face, for interfering with a legal matter where the Senate vacancy has already occurred, and the law providing for a special election without an appointment has already been triggered.

"I believe it's an untested area of the law in these specific circumstances, but I think you could make a colorable claim that we are dealing with an ex post facto law," said Tarr. "I would give you a caveat that ex post facto law generally deals with criminal law and not civil law."

That said, Tarr does see this situation as sufficiently ambiguous that it would merit examination by a court if an ex post facto challenge were made.

So let's all keep an eye on the votes in the legislature tomorrow, to see if any Dems who have opposed the law up until now end up switching and bringing it over the two-thirds margin. And eitherw ay, let's keep a close eye on any lawsuits that might get filed.