It’s always a shame to see what a Republican primary can do to someone.

Sen. Olympia Snowe (R-Maine) … stunned longtime Democratic colleagues Wednesday by blocking action on a small-business bill she co-wrote because Democratic leaders refused to allow a vote on an amendment they saw as a political hit job.

“Sen. Snowe actually wrote this bill,” said Sen. Mary Landrieu (D-La.), chairwoman of the Senate Small Business Committee. “She killed her own bill under the guise that she feels so strongly about reg reform.”

Snowe, the ranking Republican on the Small Business Committee, filibustered the bill because Democrats wouldn’t allow her a vote on an amendment she co-sponsored with Sen. Tom Coburn (R-Okla.), one of the Senate’s most outspoken conservatives. The amendment would curb federal agencies’ power to implement regulations. For example, it would require regulatory agencies to first study the potential impact on small businesses.

Now, I don’t know Snowe personally, so this is admittedly just speculation. But if she weren’t facing a tough primary challenge — and didn’t have such a low approval rating among Republican activists in Maine — would she have partnered with Coburn to block regulatory safeguards? And would she have killed her own small-business legislation, which enjoyed bipartisan backing, over a poison-pill amendment?

I kind of doubt it.

Politico added today that Senate Majority Leader Harry Reid (D-Nev.) and Snowe have had “an increasingly nasty falling out,” as the Maine senator shifts to the right in advance of her primary.

[A]ccording to Democrats, Snowe has lurched to the right and abandoned her moderate leanings. She surprised environmental activists earlier this year when she voted with Sen. Jim Inhofe (R-Okla.) and Senate Minority Leader Mitch McConnell (R-Ky.) to block environmental regulations on greenhouse gases. And she sided last month with just nine Republicans on a resolution by tea party favorite Sen. Rand Paul (R-Ky.) criticizing President Barack Obama’s policy on Libya.

Vote rankings this year show that she’s siding with her party 20 percent more often than she did in the previous Congress, meaning she has voted nine times out of 10 with a majority of her conference.

Also note, Snowe voted in March for a House Republican budget plan for the current fiscal year, which, among other things, gutted Planned Parenthood.

The right has noticed, with a Heritage Foundation staffer noting his appreciation for Snowe “voting more conservative over the past few months.” Whether the shift is enough to satisfy primary voters next year remains to be seen.

Either way, I wonder whether Snowe feels any embarrassment about all of this. She’d certainly never admit it, but the senator has been the most moderate Republican in Congress in recent years, and developed a reputation for working with Dems, being pragmatic, and having no use for her party’s extremist base.

She’s now apparently feels it’s necessary to pander to that base in order to stay in office. It’s rather painful to watch.

Steve Benen

Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.