Sen.-elect Scott Brown, R-Mass. holds up a Boston Herald newspaper during his victory party Tuesday in Boston. Republican Brown’s win of the Democrats’ 60th Senate seat has brought President Barack Obama’s health care overhaul to a defining moment.

Washington ? A one-two punch of bad news suddenly has Democrats facing an election year with campaign finance rules that favor Republicans and a Senate that can block Democratic initiatives.

Democrats already were bracing for House and Senate losses in November, which typically happens to a president’s party after his first two years in office. But a stunning GOP Senate victory in Massachusetts, and a dramatic Supreme Court ruling on political advertising, have made the horizon look even darker for the party that scored big wins in 2006 and 2008.

The week that marked President Barack Obama’s first year in office turned out to be one of the worst in recent Democratic memory.

Now party insiders are trying to figure out why public sentiment turned against them so quickly. David Plouffe, who led Obama’s winning campaign, also will play a larger role in advising the president on strategies for House, Senate and governor’s races as reeling Democrats try to rally in an important election year.

Republican strategist John Feehery says the changing sentiment began some time ago with the summertime attacks on Obama’s health care plan and continued with the GOP’s November takeover of the governorships of New Jersey and Virginia.

“It’s been bad for the Democrats for a while,” Feehery said. “They just haven’t realized it. This is the club over their head that wakes them up.”

The week’s first Democratic setback can be blamed squarely on the party’s poor performance coupled with a stellar campaign by a little-known GOP state senator in Massachusetts. Scott Brown tapped voter anger over high unemployment and unsavory dealmaking in Congress to win the seat long held by liberal leader Edward Kennedy.

Brown’s election will restore the GOP’s ability to use filibusters to block Democratic initiatives in the Senate, where Republicans will hold 41 of the 100 seats. It brought an abrupt halt to Obama’s signature issue, overhauling health care, which now hangs in limbo.

The second blow, which landed Thursday, was beyond the president’s control. The Supreme Court reversed a centurylong trend of limiting the political influence of corporations, which is likely to prompt a flood of campaign money going mostly to Republicans. The 5-4 ruling will let companies use their general treasuries, and not just employees’ limited donations, to produce and air ads for or against federal candidates.

It also applies to labor unions, which typically back Democrats. But union membership and clout have been declining for decades.

Republicans, traditionally friendly with corporate America, hailed the court decision while Democrats attacked it. Obama called it “a major victory for big oil, Wall Street banks, health insurance companies and the other powerful interests that marshal their power every day in Washington to drown out the voices of everyday Americans.”

House Republican leader John Boehner of Ohio called it “a big win for the First Amendment and a step in the right direction.”

As if the White House needed more bad news, late in the week, Federal Reserve Chairman Ben Bernanke faced mounting Senate opposition for a second term despite Obama’s support.

So far, there’s no sign of all-out Democratic panic. Party insiders anticipate about two dozen or so net losses in the House this fall. It might be worse, but few think they could lose the 40 seats that would give Republicans the majority. A GOP takeover of the Senate is even less likely.

And Republicans have their own problems, starting with less campaign money than Democrats have. Opinion polls suggest Americans have more faith in Democrats to make good decisions, and Obama’s personal popularity remains fairly high.

Also, it’s not clear that Republicans can tame and harness the volatile “tea party” activists. The fiercely independent conservatives helped Brown win in Massachusetts, but they triggered a damaging right-wing split in a special House race in New York last year.