Massachusetts Rep. Stephen F. Lynch unexpectedly took himself out of consideration for the Democrats’ Senate special election nomination Tuesday, shaking up the race just as it looked like the field for the Dec. 8 primary was solidifying.

In a short statement, Lynch said he concluded that “the challenge of putting together the resources and organization necessary to wage a competitive statewide campaign in less than 90 days is insurmountable.” Observers have estimated it will cost between $3 and $5 million to fund a competitive campaign.

The move was a surprise, as Lynch appeared poised to launch his bid in the primary to choose the Democratic nominee for the Jan. 19 special election, which will fill the seat left vacant by the Aug. 25 death of Democratic Sen. Edward M. Kennedy. Lynch had been widely expected to run as a more moderate alternative to other, more liberal Democrats in the race.

Lynch — who has represented the Boston-based 9th Congressional District since 2001 –obtained nomination papers from the office of Massachusetts’ secretary of state Sept. 4. And he filed paperwork to form a campaign committee, Stephen F. Lynch for Senate, on Friday.