There was a time when news reporters who covered politics were expected to keep their partisan leanings or preferences to themselves. That time is nearly gone.

Over the past 20 years or so, when a new generation of reporters came on the scene, the principle of maintaining a neutral façade slowly but surely began to erode.

And now, in the age of Twitter, we see political reporters increasingly not only taking advantage of the new technology to get information out to their readers faster, but also expressing their personal partisan opinions on what they are seeing and hearing out on the campaign trail.

Late last week, CNN political reporter Peter Hamby clearly showed his partisan stripes by tweeting his apparent irritation at a flurry of Tweets he must have received from Mitt Romney staffers and supporters gleefully trumpeting the front page of last Thursday’s Des Moines Register. In response, he sent out a snarky Tweet of his own, “any Romney/RNC staffers not tweeted Des Moines Register front page yet?  @KevinMaddenDC is keeping score,” he tweeted.

The Kevin Madden referred to by Hamby in the tweet is a Romney adviser and press spokesman.  All in all, it didn’t look good for Hamby’s neutrality.

The Register front page in question featured side-by-side articles on President Obama and Romney both campaigning in Iowa the day before. The pictures and headlines showed sharply contrasting images and messages:

• “Obama Sharpens Criticism,” said the headline on the Obama article.
• “Romney Expresses Optimism,” said the headline on the Romney article.

Also, the Obama picture showed the president standing in a crowd and glaring back out of the corners of his eyes. The Romney picture featured a smiling Republican challenger greeting supporters.

Not surprisingly, Romney supporters were delighted with the obviously better treatment of their candidate by the leading newspaper in Iowa, a critical swing state where the race is neck and neck.  Hamby apparently did not share their glee.

This latest flap comes on the heels of an earlier controversy in which Hamby in September was accused of partisanship when he tweeted to his followers a link to an Obama campaign donation page.

But Hamby is not alone is letting his partisanship show.  We see it quite often in Tweets by reporters reacting to on-the-spot events. It is not becoming, and hurts reporter credibility, not especially high these days. News reporters need all the reader and viewer trust they can get.

Since Hamby’s snippy Tweet, the Des Moines Register came out this weekend and endorsed Romney.  No word so far on whether Hamby was miffed by that, too. If there is a lesson in this, it is that for political reporters, Twitter should be a non-partisan zone.

And reporters should keep their politics to themselves.