POLITICAL SPOUSES have come a long way since the days when all they needed was to look adoring and fade into the background. Good thing, too, because Sheryl Lee Ralph's not the fading type. The actress, who on July 30 marked the eighth anniversary of her marriage to state Sen. Vincent Hughes, D-Philadelphia, is piling up the frequent flier miles in her latest gig, "Instant Mom," a sitcom set in Philly but filmed in Los Angeles.In the show, premiering Sept. 29 on Nickelodeon's Nick at Nite, Ralph - who was the original Deena in Broadway's "Dreamgirls," was Jennifer Hudson's mother on "Smash" and will be the subject of TV One's biographical "Life After" series on Sept. 11 - plays the outspoken mother of a new stepdaughter (Tia Mowry-Hardrict, "Sister, Sister").

As a stepmother herself, "I have to know when to keep my mouth shut," Ralph told reporters in Beverly Hills recently, then launched into an anecdote about her stepson's desire to get married when he graduates from West Point next year.

Daily News TV critic Ellen Gray caught up with Ralph there, just before the actress headed out for another flight back to Philly.

Q So you've been commuting?

I get lots of miles going back and forth. Sometimes as soon as we finish, I'm back on the plane.

Q You didn't just create a blended family: You married a politician. That's a story in itself.

Yes, it is. When I met my husband, the head of diversity from Paramount was a friend of both of ours, and she said, "The two of you have to meet." And I was like, "Yeah. Your friend, the senator. Right."

And then we met after a year or so. He had a hat on, right? And then he took off his hat and he was follicle-y challenged. Then he took off his jacket and I'm like, "Oh, no. I'm going to have to run 30 pounds off of him in the gym."

But when I looked at him, I was like, "Oh my God, I'm smitten."

Q Isn't a commuter marriage a challenge?

Yes, it is. But we don't share a mortgage, a bathroom and we raise our own kids. We never argue and every time he sees me, he's happy to see me. We are on a perpetual date.

We bought a house in Wynnewood. We're getting ready to redo that - with series money, yeah! - so God has blessed me richly and greatly. And I look at my husband and I realize how he has been able to love me just as I am, with all the things I do, and love my children to the best of his ability as if they were his own. That can only be from God.

Q Does your character live with her daughter's new family?

You would think so! I'm sort of like my own mother-in-law: I'm always there. And I always have something to say. I'm a little bit bigmouthed, but at least you understand exactly what I'm saying and you know I say it with love and wisdom.

Q And is this good practice for being a mother-in-law yourself someday?

No. Because if I had a mother-in-law like this, I might strangle her.