Rep. Bart Gordon (Tenn.) on Monday became the fourth House Democrat from a competitive district to announce a retirement in as many weeks, further energizing Republicans and putting Democrats on high alert, with several party strategists conceding that more departures are inevitable.

Republicans held up the late-year retirements as just a hint of what awaits the Democratic majority heading into the 2010 midterms, as nervous incumbents face a much tougher political environment than in 2006 or 2008.

“It’s official: Democrats now have a retirement problem,— declared Ken Spain, communications director of the National Republican Congressional Committee. Keeping up the party’s drumbeat of opposition to the agenda pursued by President Barack Obama and Speaker Nancy Pelosi (D-Calif.), Spain continued, “This is evidence of the fact that the Obama-Pelosi agenda of government takeovers, permanent bailouts, and fewer jobs is taking a political and mental toll even on incumbent Democrats who were once perceived to be firmly entrenched.—

House Republican leadership aides touted Gordon’s retirement as a victory. “The more moderate Democrats that bow out, the greater the chance we have of picking up seats,— a senior House GOP aide said.

House Democratic leadership aides said that they expect more retirements soon, but only because the year is almost over and the House has already passed its top three priorities for the year: economic stimulus, climate change and health care reform legislation. Several older, long-tenured Members are being eyed as possible retirees in 2010, including Budget Chairman John Spratt (D-S.C.) and Armed Services Chairman Ike Skelton (D-Mo.).

Some Democrats, though, stressed that they aren’t ready to hit the panic button yet.

“Our major hurdles have been crossed. It’s the right time— for more retirements, one senior Democratic aide said. “We’ve moved the agenda in the House.—

Steve Murphy, a longtime Democratic consultant and principal in the Virginia-based firm Murphy Putnam Media, noted that “it is the retirement season,— the point in the election cycle at which departing incumbents often announce their intentions.

Murphy said recent retirees such as Gordon, fellow Tennessee Rep. John Tanner and Washington Rep. Brian Baird were popular figures back home who would have weathered any difficulties produced by the national political atmosphere in 2010.

“They were not being chased out by the election year,— Murphy said.

Persuading incumbents to run again in 2010 is crucial for House Democratic leaders. They know that the party holding the White House almost always loses seats in a midterm election. And they know that avoiding a slew of open seats — which are harder to hold than those defended by incumbents — may be the difference between a moderately difficult election year and one in which Republicans could seriously threaten to erase the Democrats’ 41-seat majority.

GOP strategists are convinced they will be able to at least trim the Democrats’ majority by ousting junior Members who captured Republican seats in their party’s 2006-08 upswing. But the net gain of at least 41 seats that Republicans need to capture a majority is daunting. The last time a party swallowed that many seats in one gulp was in 1994, when the Republicans scored a 52-seat gain.

So the Republicans will need to pad their target list with seats held by longer-serving Democrats. And the Republicans’ takeover chances improve when a veteran Member voluntarily decides that it’s time to go.

The NRCC has been ratcheting up attacks on Democratic incumbents whom they see as potential retirees, and the decision by 13-term incumbent Gordon to step aside may be seen by Republicans as a sign that their strategy is bearing fruit. Democrats employed the same strategy in the last two election cycles, attacking senior Republicans in swing districts in the hopes of chasing them into retirement. The strategy worked in a few places.

As chairman of the House Science and Technology Committee, Gordon is the highest-ranking among the quartet of recent Democratic retirees. The list also includes six-term Rep. Dennis Moore of Kansas, chairman of the Financial Services Subcommittee on Oversight and Investigations; 11-termer Tanner, chairman of the Ways and Means Subcommittee on Social Security; and six-term incumbent Baird, chairman of the Science and Technology Subcommittee on Energy and Environment.

These departures raise uncomfortable questions for Democratic leaders about why Members who endured years in the minority are checking out not long after obtaining their committee or subcommittee gavels.

But the more immediate problem for the Democratic Congressional Campaign Committee is that these retirements all create open-seat headaches. The two Tennessee seats are located in conservative-leaning districts that favored Sen. John McCain (R-Ariz.) for president last year while electing Democrats to the House. Gordon’s 6th district gave McCain a whopping 62 percent of its votes, while Tanner’s nearby 8th gave McCain 56 percent.

Obama narrowly carried Moore’s home base in Kansas’ 3rd district with 51 percent, and won Baird’s base in Washington’s 3rd with 53 percent. But Kansas’ 3rd district has an overall Republican lean, Washington’s 3rd is a swing district, and the NRCC has placed both high on its target list. Democrats appear to have solid candidates in the Baird and Tanner districts but have yet to recruit anyone into the Kansas race.

In addition to Spratt and Skelton, other Democratic incumbents in competitive districts who will now be closely watched for retirement jitters include Reps. Marion Berry and Vic Snyder of Arkansas, Rep. Baron Hill of Indiana, and Rep. Paul Kanjorski of Pennsylvania. Berry, Snyder, Skelton, Hill and Spratt all represent “McCain-Democratic— districts. Kanjorski’s district is more strongly Democratic, but he faces a probable rematch with a Republican candidate whom he held off by just 3 points in 2008.

Democrats have an additional worry in Hawaii’s 1st district, where Rep. Neil Abercrombie (D) is resigning soon to run for governor. Although the district is reliably Democratic, all of the candidates in the special election to replace him will appear on one ballot, meaning the leading Republican candidate, Honolulu City Councilman Charles Djou, could sneak in if top-tier Democrats split the Democratic vote.

It is premature, though, for Republicans to count on a wave of Democratic retirements. In the 2008 election cycle, just 11 of the 32 House Members who did not seek re-election made their announcements after Dec. 10, 2007.

Gordon’s retirement made him the 21st House Member not seeking re-election, including 17 who are running for other offices. That is identical to the number of departures announced by the same point in the 2007-08 cycle.

Democrats are not suffering the kind of imbalance in open seats that the GOP endured in 2008, when there were 26 Republican open seats to just six for the Democrats. So far, there are more Republican open seats (12) for 2010 than Democratic (nine) — though all the Republicans who are leaving the House so far are running for other offices in 2010.

And the Republicans have retirement worries of their own. They include septuagenarian Reps. Bill Young of Florida, Henry Brown of South Carolina and Vernon Ehlers of Michigan, all of whom represent potentially competitive districts. A handful of GOP-held seats that Republicans are giving up to run for other offices also are in danger of flipping to the Democrats.

But for now, it is the Democrats who seem more at risk of losing open seats.

“We don’t need a lot more Members retiring in swing districts,— Murphy said.

Jennifer Bendery and John McArdle contributed to this report.