WASHINGTON — As Rep. Tammy Duckworth, D-Ill., moves toward challenging Sen. Mark Kirk, R-Ill., in 2016, three of her Democratic House colleagues also are mulling a bid, with Rep. Robin Kelly the latest name to surface.

The field so far: Duckworth, Kelly and Reps. Bill Foster and Cheri Bustos.

Not in the game: Attorney General Lisa Madigan every two years flirts with running for senator or governor, but never does. Madigan’s political director, Michelle Young, told me on Friday a 2016 Senate bid is “unlikely.”

Duckworth, the frontrunner, is getting more serious about a run as she wraps up her maternity leave next month. Her daughter, Abigail, was born Nov. 20, a few weeks after Duckworth won a second term to represent the northwest suburban 8th Congressional district.

As a new mother, Duckworth understandably is taking her time about making a decision. She has a good chance to clear the Democratic primary field if she runs.

Everyone wants to know what Duckworth will do.

As a wounded Iraq war vet and a former top official in the Illinois and U.S. veterans administrations, Duckworth has vastly more stature, fund-raising potential and name recognition than her House rivals who are putting themselves in play.

Kelly told me on Friday, “I am doing due diligence to see if there is a pathway to victory.” Kelly also said, “I am more in the exploring stage,” as she is speaking to “longtime friends.”

Would she run against Duckworth? Kelly said, “I will run if I see a pathway to victory for myself.”

Kelly was sworn in April 9, 2013, in a special election to replace the now imprisoned former Rep. Jesse Jackson Jr. in the 2nd Congressional District, anchored in the southern suburbs. She easily won re-election last November in the overwhelmingly Democratic district.

Of the four Illinois House Democrats, Kelly, a former state representative, is the only African-American and the only one to have run statewide. In 2010, she won the Democratic nomination for treasurer, but lost in the general election to Republican Dan Rutherford.

Kelly has a Downstate connection: A graduate of Bradley University, she lived in Peoria and remains involved with the school.

Foster was first elected to the House in 2008, was defeated in 2010 and made a comeback in 2012, winning re-election to the west suburban 11th Congressional District seat in 2014.

In the past weeks, Foster has been making calls to potential supporters about a Senate bid.

And in what I took as a demonstration of growing intent to run for the Senate, last week Foster and his former wife discussed their 1996 divorce with Sun-Times columnist Carol Marin in order to bury allegations contained in their divorce file that he caused her “physical abuse and emotional harm.”

Bustos represents the 17th Congressional District in northwest Illinois, first elected in 2012 and winning a second term last November, staking a deeper claim to a seat in a tough swing district.

She is the only Downstate Democrat left in the Illinois delegation and out of necessity has become a strong fund-raiser.

“Congresswoman Bustos is focused on advocating each day for Illinois’ 17th District, and in that respect is open to any opportunity that would enable her to best advocate for the hard-working people of her region,” her spokesman, Colin Milligan, told me.

This will all sort itself out. I don’t expect all four Illinois House Democrats will give up their seats to carve each other up in the March, 2016, Democratic primary. Nominating petitions will be due at the end of November 2015.

Anyway, it will be a giant headache for the Democratic Congressional Campaign Committee — the House Dem political shop — if they have to recruit replacement candidates for Bustos and Foster. If they step down, they put Democratic control of their districts at risk.

A lot of this is positioning to be the Duckworth alternative if she decides not to run.

“I genuinely like my job,” said Rep. Mike Quigley, D-Ill., whose goal is to move up in the House.

Said Quigley, “I think this far out, you may not have heard from all the [Senate] candidates. Did anyone know who Bruce Rauner was two years ago?”

Rumblings: A close confidant of former Gov. Pat Quinn told me “his people” have not dismissed the idea that Quinn would mount a Senate bid. His spokesman, Brooke Anderson, told me, “That’s not something that’s on Gov. Quinn’s radar right now.”