The recent uproar over retirement announcements by two Democratic senators and a governor may pale beside the likely storm if a little-known Republican captures Edward Kennedy's Massachusetts Senate seat next week.

For starters, it might doom President Barack Obama's top legislative priority, his health reform bill, by costing Democrats the 60-vote majority they need to win Senate approval.

Though the race has gotten closer, most polling and pundits still see Democrat Martha Coakley holding off Republican Scott Brown in the heavily Democratic state. That would keep the party's 60-vote Senate majority long enough to approve the health bill and some other Obama initiatives, including possibly another Supreme Court nomination next summer.

But that Senate margin — and the Democrats' 79-seat cushion in the House — seems unlikely to survive November's elections. Indeed, the only real question is whether Democratic losses will be modest or severe.

Recent speculation notwithstanding, however, mid-term elections are notoriously hard to predict this far out. And the facts that have historically explained such contests present a somewhat mixed picture.

One historical pattern — modest losses for the party in power after two years and bigger ones after six — was overturned during the Clinton years. Republicans scored sweeping successes two years after Bill Clinton's election, but Democrats rebounded somewhat after six years.

Republicans hope to repeat their 1994 victory, but three factors that contributed to that result don't seem present this year:

• Democrats suffered from Clinton's failed effort to enact health reform, and votes for a ban on assault weapons hurt some members from rural districts. In 2010, Democrats expect to benefit from anticipated passage of health reform, despite mixed attitudes toward the current proposals.

• In 1994, Democrats lost all of their six open Senate seats and 22 of 28 open House seats. In 2010, Republicans have slightly more open seats: 6-4 in the Senate and 12-11 in the House.

• GOP gains in 1994 may have been a delayed benefit from redistricting built on the 1990 census, which President George H.W. Bush's weakness and Ross Perot's independent candidacy prevented us from seeing in 1992.

This year, Senate Democrats have both fewer open seats and a bigger majority, 58-40, than the 56-44 in 1994. Even the loss of a half-dozen seats, more than most analysts expect, would leave them with a clear majority.

In the House, a big Democratic problem is that 49 members represent districts Republican presidential nominee John McCain carried in 2008, compared with 34 Republicans from pro-Obama districts.

The turnout of younger and minority voters for Obama aided some of 21 Democratic House gains in normally Republican districts in 2008. Historically, those kinds of districts are vulnerable in mid-term elections. One reason Republicans lost 27 seats in 1982 was that President Ronald Reagan helped them gain 35 in 1980, many in normally Democratic areas.

Conversely, John F. Kennedy and George W. Bush suffered minimal House losses in 1962 and 2002 because their parties didn't gain seats two years earlier.

Undoubtedly, the two biggest threats to Democrats are the state of the economy and its impact on Obama's standing. While presidents don't have coattails in mid-term elections, the national mood and a president's popularity can contribute to his party's success, especially in generating partisan turnout.

One factor in last November's Democratic gubernatorial setbacks in New Jersey and Virginia was a sour mood and diminished enthusiasm among Democrats. All current signs suggest Republican critics of Obama are more enthusiastic about voting this year than his Democratic supporters.

That pattern will hurt Democrats if it persists into the fall, something that depends on whether the economy improves and Democrats benefit from passing health reform, assuming the Massachusetts election doesn't derail it.

And that could decide if Democrats hold GOP gains to a relatively modest level or suffer a severe setback.