House retirements have set up a quartet of hard-fought expensive open-seat Democratic primaries, and the latest fundraising reports released this week suggest that each of them is very much up for grabs.

All four primaries are in reliably Democratic districts, meaning the primary outcome is almost certain to determine who the next Member of Congress will be, even though Republicans are suggesting that they will compete in one or two of the races.

Perhaps the most significant of the second-quarter fundraising results was in Colorado’s 2nd district, where Rep. Mark Udall (D) is moving on to run for an open Senate seat in 2008.

From April 1 to June 30, the nominal frontrunner in the race, state Senate President Joan Fitz-Gerald (D), was outraised by both of her primary opponents, former state Board of Education Chairman Jared Polis and environmentalist Will Shafroth.

Fitz-Gerald’s take for the three-month period was respectable enough: She raised $236,000 — including $15,000 from her own pocket — and banked $189,000. But some insiders consider that total a little disappointing, given Fitz-Gerald’s institutional power in the state Capitol, her status as the candidate of the Democratic establishment and her support from women’s groups.

Polis, a millionaire Internet entrepreneur who is in his mid-30s, seeded his campaign with $155,000 of his own money and reported taking in $457,000 overall. Even without his own contribution, Polis raised more than Fitz-Gerald — $302,000. He finished June with $403,000 in the bank.

Shafroth, a well-respected environmental leader in Colorado, raised $301,000 and ended the period with $288,000.

EMILY’s List, the national fundraising powerhouse, is widely expected to endorse Fitz-Gerald at some point during the cycle, but the group has gotten behind only Congressional incumbents, special election candidates and the presidential campaign of Sen. Hillary Rodham Clinton (D-N.Y.) to date. If Fitz-Gerald supporters sense that she needs financial help soon, EMILY’s List may issue an endorsement sooner rather than later.

Meanwhile, in Illinois’ 4th district, Chicago Alderman Manny Flores (D) has jumped out to an early fundraising lead in the race to replace retiring Rep. Luis Gutierrez (D). Flores raised $478,000 from April 1 to June 30 and banked $436,000; Cook County Commissioner Ricardo Munoz (D) collected $310,000 and had $308,000 on hand; and Alderman Roberto Maldonado (D) took in $183,000 and finished the period with $167,000.

The field in the Chicago-area race could grow to include state Rep. Susana Mendoza (D) and Alderman Danny Solis (D), the brother of Clinton campaign manager Patti Solis Doyle.

Of the four primaries, the least money is being spent so far in Maine’s 1st district, where Rep. Tom Allen (D) is moving on to challenge Sen. Susan Collins (R). But the fundraising leader there is a woman who has spent the past several years fighting the influence of money in politics, former Common Cause President Chellie Pingree (D).

Pingree, a former state Senate Majority Leader who built a fundraising network when she unsuccessfully challenged Collins in 2002, raised $233,000 and finished the quarter with $162,000.

Two other former state Senate leaders also are in the race: York County District Attorney Mark Lawrence (D), a former Senate president, raised $171,000 and banked $127,000; former Majority Leader Michael Brennan (D) raised just $63,000 and had $56,000 on hand. Iraq War veteran Adam Cote (D) raised $114,000 and ended June with $99,000 in the bank.

Money is flowing a lot faster a short hop down the interstate, in the special election to replace ex-Rep. Marty Meehan (D-Mass.), who resigned July 1 to take a university post.

In the Lowell-based 5th district, college administrator Niki Tsongas (D) — the wife of the late Sen. Paul Tsongas (D-Mass.) — is the perceived frontrunner, and she leads on the financial front as well. Aided by endorsements from certain Members of the Massachusetts Congressional delegation and EMILY’s List, Tsongas raised $670,000 in the quarter and left $563,000 in the bank.

But other Democrats also are performing at a high level financially. State Rep. Barry Finegold raised $414,000 and banked $450,000. Former Lowell Mayor Eileen Donoghue took in $159,000 and had $279,000 left over, though the latter figure is supplemented by $325,000 she loaned her campaign in the first quarter of the year. State Rep. Jamie Eldridge raised $158,000 and ended June with $141,000 in the bank.

The special Democratic primary is Sept. 4, and the general election to fill the remainder of Meehan’s term will be Oct. 16.