A couple of weeks after persuading Rep. Steve Israel (D) not to challenge appointed Sen. Kirsten Gillibrand (D) because he worried that a bloody primary would hurt the Democrats’ ability to hold on to the New York Senate seat, President Barack Obama sent a letter to his Virginia supporters on behalf of Creigh Deeds, the new Democratic gubernatorial nominee in the Old Dominion.

[IMGCAP(1)]“I know a thing or two about tough primaries, and I know a thing or two about running tough races in Virginia,— Obama wrote. “Creigh Deeds’ victory once again shows the power of people at the grassroots to win elections and bring about lasting change — and he will be a better candidate this fall because of his hard work to win the nomination.—

So, Mr. President, Democratic primaries are good in Virginia but not in New York?

Obama leaned on Israel partly at the behest of Sen. Charles Schumer (N.Y.), a master at shaping Democratic primaries during his two cycles as chairman of the Democratic Senatorial Campaign Committee. That’s right — the same Chuck Schumer who freely admits that his own tough three-way Senate primary in 1998 served as a terrific tuneup for the general election, when he ousted three-term Sen. Al D’Amato (R).

So which is it, fellas? Are primaries a good thing or not?

It’s fair to say that without primaries, there wouldn’t be a President Obama right now. Then-Sen. Hillary Rodham Clinton was supposed to own the Democratic presidential primaries in 2008. Her handlers were so confident about the primaries, in fact, that they ran a general election strategy. Not a good idea.

Take this a step and four years further, and it’s also fair to say that without a primary, there wouldn’t have been a Sen. Barack Obama — at least not a U.S. Senator. When Obama launched his 2004 Senate campaign, few people could imagine a skinny guy with a funny name cobbling together a coalition of African-Americans and white do-gooders to topple a multimillionaire and the candidate of the Democratic machine. No primary, no Obama — he’d be stuck in Springfield, or teaching full time, or settling uneasily into a gig at a Chicago law firm, instead of leader of the free world.

Schumer himself all but admits that without the 1998 primary, he wouldn’t be a Senator now. He’d be going crazy in the House, or searching for another office to run for, getting his greatest satisfaction from the fact that he had long outlasted his arch-rival from Brooklyn, ex-Rep. Stephen Solarz (D).

The smart Democratic money in 1998 was on Geraldine Ferraro, but Schumer refused to accept the verdict of the bosses and wound up beating Ferraro by 25 points in the primary. He came out of that contest with so much momentum that D’Amato was powerless to stop him.

So primaries are good, right? Except when they’re bad. Guess the Obama-Schumer message is, do as we say, not as we do.

Is there an official Democratic line on primaries these days?

“In general, we certainly prefer to avoid primaries,— says Eric Schultz, a DSCC spokesman.

A top Democratic strategist with a line into the Obama White House dredges up the late Supreme Court Justice Potter Stewart’s famous observation that he used when discussing pornography: “I know it when I see it.—

“You don’t know the impact of a primary until it’s over with,— this strategist says.

OK, fair enough. The pros and cons of primaries are familiar to everyone who follows politics even a little. On the good side, they test candidates for the wars to come and in many cases serve to fire up the activists. On the bad, they can be incredibly nasty and a drain on resources. They help the opposition amplify lines of attack that have already been used.

Primaries have certainly vexed party leaders and the campaign committees in the recent past.

In the 2008 cycle, for example, National Republican Congressional Committee Chairman Tom Cole (Okla.) took a hands-off approach in most GOP primaries and the party paid dearly in three straight special elections, losing Republican-held seats because their nominees were so flawed.

Schumer showed a deft if heavy hand as DSCC chairman, and the proof of his effectiveness was evident in the 14 seats the Democrats flipped in 2006 and 2008. In an interview last week with the Village Voice, Schumer boasted of the “gut decisions— he had to make to find the strongest candidates, and he noted that there had been “a whole lot of damaging [Democratic Senate] primaries— before he took over at the committee.

Still, according to the Voice article, bitterness lingers among those who lost primaries to Schumer’s handpicked candidates and those who were chased away — especially former Pennsylvania state Treasurer Barbara Hafer (D), who got into the 2006 Senate race at the behest of Keystone State Democratic leaders, only to be pushed out when Schumer came out strongly in favor of Bob Casey (D), who had a famous name and opposed abortion rights. Schumer conceded in the Voice interview that the man Casey defeated, then-Sen. Rick Santorum (R), was such damaged goods by 2006 that Hafer, who supports abortion rights, would also have beaten him.

But that was then. Now Schumer’s successors at the DSCC, along with Obama, seem intent on preventing primaries for Gillibrand and newly minted Democratic Sen. Arlen Specter (Pa.).

That’s not surprising. Party leaders almost always come out for incumbents in primaries, except when they don’t (see Bunning, Jim). It’ll be interesting to see how far the party bosses go to clear the field in New York and Pennsylvania.

A somewhat more delicate dance is taking place in Illinois, where D.C. Democrats are trying to persuade state Attorney General Lisa Madigan to run for Senate and not for governor in 2010. This enables another Senate candidate, state Treasurer — and erstwhile Obama basketball playing buddy — Alexi Giannoulias (D) to inveigh against the “Washington insiders— who are trying to deny Illinois voters a choice. It’s funny how the beleaguered incumbent Senator, Roland Burris (D), isn’t even a part of the discussion at the moment.

More delicate still is what may be happening under the radar in Connecticut. Right now, Obama is committed to helping shaky Sen. Chris Dodd (D) win a sixth term. But if Democratic leaders become convinced that Dodd cannot survive in 2010, who delivers the news?

But perhaps the most interesting primary dilemma this cycle for Senate Democrats — and the least talked about — could develop in a place where they think they have an excellent shot of picking up a seat next year, Kentucky.

Right now, national Democrats seem content to let their two Senate candidates there, Lt. Gov. Dan Mongiardo and state Attorney General Jack Conway, slug it out. Both are young and attractive and both, according to polls, would slaughter Bunning in the general election.

But what if Bunning moves on, as national Republican leaders so desperately hope he does? Although a competitive primary could emerge on the GOP side, most insiders will fall in line behind Secretary of State Trey Grayson — meaning most of the drama will then switch to the Democratic side.

What will Democratic leaders do then?

What would Potter Stewart do?