That confirmation will set off a special-election fight later this year for his seat, and a number of prominent Massachusetts Democrats, including Reps. Michael Capuano and Stephen Lynch, have expressed interest in running, though neither has launched an official bid.

But national and Massachusetts Democrats lined up behind Markey shortly after he announced his bid, with Democratic Senatorial Campaign Committee Chairman Michael BennetMichael Farrand BennetHickenlooper: Law preventing cannabis business banking 'a recipe for disaster' Senators call for Smithsonian Latino, women's museums to be built on National Mall Sununu exit underscores uncertain GOP path to gain Senate majority MORE (D) endorsing him in what essentially marked the national Democratic Party throwing its weight behind Markey. 

Victoria Kennedy, widow of the late Sen. Edward Kennedy (D), also endorsed Markey, and Kerry himself offered effusive praise for Markey, though he stopped just short of officially endorsing Markey for the seat.

The quick outpouring of support from the trio was largely read as an effort by Democrats to clear the field and provide for an easier nominating process for Markey to avoid a potentially bruising primary that could occur if other candidates jump in the race.

Outgoing Sen. Scott Brown (R), who was defeated this cycle by Warren, is the likeliest GOP nominee for the seat and has dropped hints that he plans to run, though he hasn't yet made it official. But Warren wouldn't say whether she'd be willing to back any Democratic candidate in the race that could turn her former opponent into her colleague.

"Right now, Congressman Markey is the one who has declared that he is in this race and, as I said, he would make a great senator," she said when asked whether she planned to endorse at all in the race.

Her endorsement would be a significant boost for any Democratic candidate running against Brown, and she would be a strong asset on the trail if she chose to campaign for any candidate.

Lynch recently said he was still interested in the seat, and many Massachusetts Democrats believe the sudden endorsements for Markey could come across as an unwarranted coronation, sparking Capuano to launch a run.

But Warren offered equal praise to both Capuano and Lynch when pressed on whether she'd be willing to jump in the race and back one candidate.

"Congressman Capuano and Congressman Lynch have also served in Congress and served the people of Massachusetts with great distinction, and each of them would make great senators," she said.