Sen. Arlen Specter said after his switch to the Democratic Party last week that he would not be a "party-line voter" or "an automatic 60th vote" to head off a Republican filibuster. He put an exclamation point on those pronouncements by voting against President Obama's $3.5 trillion budget.

I know Arlen Specter. Arlen is a friend of mine. And I believe him when he says he will be as much of a pain to Democratic leaders as he was to Republican leaders.

Specter cherishes the label independent. He sought it because the media loved it, and that led to November victories.

Now let's see how many sweet nothings the media whisper in his ear when he stiffs their president.

It appears that after 29 years with a conservative rating of 44 percent (the lowest among Republican senators) and numerous high-profile breaks with the GOP (opposing Robert Bork's nomination, importing Scottish law into Bill Clinton's impeachment), Specter finally broke the elephant's back by supporting Obama's unprecedented $787 billion stimulus package. And so Pennsylvania's ultimate political survivor did the only thing he knew to do and jumped on the donkey's back to fight another day.

According to Specter and a herd of commentators - many in The Inquirer's pages - the senator's switcheroo is proof of far more than Specter's brute survival instinct; it's proof that the Republican Party is narrow and rigid.

Keep spinning. The fact is that, prior to the stimulus vote, conservative former Rep. Pat Toomey was telling supporters that the chances he would challenge Specter in the 2010 Republican primary were 50-50 at best. The Senate's most liberal Republican would have been the heavy favorite to win the nomination again.

So now we're to understand that Specter has left the small-tent GOP for the broad-minded, tolerant Democratic Party? Alas, voting records tell a different story.

According to the American Conservative Union, in 2008, only two of the 51 Senate Democrats had conservative ratings above 25 percent. But 21 of the 49 Republican senators had conservative ratings of 75 percent or less. Furthermore, 12 Democrats had a conservative rating of zero, while an additional 25 scored under 10 percent. By contrast, only one Republican scored a perfect 100, and a scant seven others scored above 90 percent. (Note that Pennsylvania's other "moderate" senator has a conservative rating of 8 percent.)

Which party has greater ideological diversity? Sorry, but the facts just don't fit the media's fantasy.

I spent 12 years in the Senate. It has one doctrinaire, narrow, intolerant caucus, and Specter just joined it.

The Democratic caucus is a reflection of the leftist special interests that control the Democrat Party. From 1994 to 2004, those interests supported liberal candidates who were too far out of the mainstream to win in competitive states and districts. They regained power in Pennsylvania and elsewhere because Democratic candidates ran as moderates or conservatives, and Republicans abandoned the principled positions that had helped them win.

The public can tell when it's being conned, so it chose to throw out the party that had lost its way. The result: the most hard-left Congress in U.S. history (see previously mentioned ratings).

As demonstrated by his new Democratic colleagues' decision to strip him of his all-important seniority, Specter will find that being independent won't be as easy as it was with his old GOP brethren. He may soon wish that were the only difference.

Obama and former Specter employee Gov. Rendell will do all they can to clear the Democratic field for the senator. But the liberal special interests that have a huge say in selecting Democratic nominees will be less forgiving of the man who would be the most conservative Democrat nominated to statewide office since Pete Flaherty, whom Specter defeated in 1980.

Enter the former vice admiral and Democratic Rep. Joe Sestak (conservative rating: zero), who has made it clear he has his periscope trained on Specter. Sestak is intimating that he will launch for the port side of the USS Specter if it strays off course. When the ship last sailed, in 2004, it got only one in five Democratic voters on board.

And then there is the threat of any of three formidable Republican destroyers - Tom Ridge, Pat Toomey, or Pat Meehan - in the general election.

Safe sailing, Arlen.

Former Sen. Rick Santorum (R-Pa.) is a senior fellow at the Ethics and Public Policy Center.