While watching Hardball last night, I became convinced that Chris Matthews is indeed positioning himself for a U.S. Senate bid from Pennsylvania - simply because of the way he was pulling his punches on the air.

Some observers argue that Matthews has been using his MSNBC forum to butter up the key politicians whom he would need in his corner during a Senate campaign against incumbent Arlen Specter. Case in point, his relentless slobbering over Gov. Ed Rendell. (Nov. 4: "You're the best political analyst in Pennsylvania, governor." Oct. 23: "You're the best pol in the state." April 2: "I think Eddie Rendell is the smartest politician in the state, as we know." Feb. 13: "One the smartest people in politics.") Although I suppose that, as a counter-argument, it's worth noting that Matthews lavishes this kind of praise on many of his guests - including the journalists, many of whom are lauded as preeminent sages.

No, what concerns me - and this should also concern MSNBC - is when he appears (and perception is important) to fall silent for his own partisan ends.

Last night, for instance, he spent considerable time recounting the latest developments in the Rod Blagojevich debacle in Illinois - which, as he accurately noted, is a classic case of "pay to play" corruption (politicians skewing their decisions to benefit those who have ponied up the campaign cash). He then remarked, several times, that "pay to play" is not just a Chicago phenomenon, that in fact the practice has been common in other cities. He didn't, however, mention any specific cities.

Indeed, I kept waiting for him to cite the most obvious recent example, the most journalistically valid example, but he never did:

The city of Philadelphia.

On his show, Matthews often uses any excuse to talk about his native city - the politics, the pols, the wards. But, curiously, not this time.

Back in 2003, Philadelphia was rocked by an going FBI probe of "pay to play" practices in City Hall, under the regime of Mayor John Street. The FBI even planted a bug in Street's office. The whole thing dragged on for a couple years. In the end, the probe led to more than a dozen indictments and 10 convictions. Matthews could have booked any number of talking heads to recount the Philadelphia experience (such as the Committee of Seventy civic watchdog group), and he could even have noted that, in a sense, the scandals ended happily - with the passage of ethics reform (via referendum, in 2005), and the election of a reform-minded mayor in 2007.

Instead, Matthews said nothing about Philadelphia.

I happen to be a Matthews fan, if only because his authentic enthusiasm for politics is so infectuous, and because the guy on camera is the same guy off camera. My favorite encounter with Matthews was in June 1999, when we and other journos were up in Maine early one dewy morning, awaiting the arrival of newly-announced candidate George W. Bush; while shooting the breeze, Matthews told me that, when he was a kid growing up in Northeast Philadelphia, he dreamed of becoming either the editorial page editor of The Philadelphia Inquirer, or the Philadelphia police chief. And I've since been a guest on the show a handful of times.

But this omission of Philadelphia from the discussion of "pay to play"...well, that just didn't look good, at least in journalistic terms. One can easily imagine that the last thing a prospective Democratic senatorial candidate from Pennsylvania would want to do is tick off the major Democratic players in the most populous Democratic city. And such a prospective candidate would indeed risk ticking off these major players (starting with Rendell and Bob Brady), by bringing up "old business" that puts the city in a bad light. So the prudent strategy, it appears, was to say nothing.

Prospective candidates make these kinds of calculations all the time. The difference here is that this prospective candidate hosts a national TV show that is expected to cover politics without any hint of self-interest.

Obviously, nobody can channel Matthews and determine whether he went mute on Philadelphia simply to guard his political options. But it is easy to perceive it this way, and that's his growing problem. As Democratic operative Phil Singer asked on his blog earlier this month, "If Matthews is going to run as a Democrat in what will likely be a contested primary, will he be willing to play hardball when his fellow Democrats are in the news?...More to the point, will viewers think he is covering politics without fear or favor."

At some point soon, Matthews will need to make his intentions perfectly clear, either by giving up the candidacy, or giving up the show. Because the longer he sticks with the latter while exploring the former, the more he risks losing credibility with his loyal viewers.