Sen. Arlen Specter (R-Pa.) began the second phase of his quest for an unprecedented fifth term Wednesday, as the Republican establishment issued a collective sigh of relief and Democrats sought to mask their disappointment over having to face the bruised, yet still entrenched, 24-year incumbent in November.

In Tuesday’s GOP primary Specter narrowly beat back an insurgent challenge from Rep. Pat Toomey, a three-term lawmaker who made ideology the centerpiece of his campaign.

Tuesday’s balloting in the Keystone State also set the stage for several competitive House races this November, in one of the only states where the presidential, Senate and House battlegrounds fall into overlapping territory.

Specter defeated Toomey 51 percent to 49 percent, as voter turnout was light across the state. He now faces a showdown with Rep. Joe Hoeffel (D) in a race that Democratic strategists have insisted will be critical to regaining a Senate majority.

Despite being forced to defend five open seats in the South, the party’s prospects for getting to 51 have brightened in recent months and strategists had been even more optimistic with Toomey’s late rise in the polls.

Democrats’ chances of picking up a seat in Pennsylvania would have been greatly bolstered by a Hoeffel/Toomey matchup — and President Bush’s efforts to win Pennsylvania may also have been jeopardized. Instead, Democratic strategists spent much of Wednesday painting Specter as a heavily damaged incumbent and making the case for why the race is still winnable.

Tuesday’s “results, where a four-term incumbent came within a whisker of being turned out of office by his own party, proves that Arlen Specter is vulnerable and the people of Pennsylvania are ready for a change,” said Democratic Senatorial Campaign Committee spokesman Brad Woodhouse. “Being rejected by nearly half of the voters of your own party after such a long career is devastating and is no mandate to take into the general election.”

Democrats were also quick to note that the last time Specter faced re-election in a presidential election year, he squeezed by Democrat Lynn Yeakel 49 percent to 46 percent in 1992, despite outspending her 2-to-1.

The DSCC plans to launch a new Web site today, www.spectertwostep.com, highlighting a phrase Toomey used during the only primary debate to attack Specter’s varying positions.

“He had to move far to the right to narrowly defeat Pat Toomey — now he will try in the most unprincipled fashion to move back to Pennsylvania’s moderate middle in the general election,” the site reads. “This site is designed to keep him from getting away with it.”

Republicans, meanwhile, immediately moved to unify the party on Wednesday, with Toomey and his most prominent backer both pledging to help re-elect Specter this fall.

Club for Growth President Stephen Moore, whose organization poured more than $2 million into the effort to defeat Specter, said in a statement that Specter deserves the support of all Republicans in November.

“Although we disagreed with Arlen Specter on a number of economic issues, there is no doubt that Senator Specter is far more likely to support tax cuts and other Bush economic policies than Representative Hoeffel,” Moore said. “We have no hesitation in endorsing Arlen Specter for Senate.”

Specter was boosted heavily by the unified support of the party’s establishment in the primary, most importantly President Bush and Sen. Rick Santorum (R), a prominent conservative in the Keystone State.

The National Republican Senatorial Committee shot down Democratic efforts to portray Specter as vulnerable, issuing a statement that cited his primary win as “another major pothole in the Senate Democrats’ ‘path to 51.’”

“I think that the Democrats are trying to put the best face on this situation, but the facts are clear they’re not going to beat Arlen Specter in November,” said NRSC spokesman Dan Allen. “They have a candidate who is underfunded, compared with what Arlen Specter starts off the general election with, and Specter will grow stronger and stronger as we move closer to November — everyone knows that.”

Specter begins the general election with an estimated $2 million in cash on hand, after spending upwards of $12 million on the primary. Hoeffel, meanwhile, showed just $800,000 in the bank as of the early April pre-primary filing.

Privately some Democrats concede that Hoeffel’s fundraising has been hampered by Specter’s presence in the race. Both men share the same Southeastern Pennsylvania base, and Specter enjoys strong backing from organized labor.

Also at issue now that Specter has cleared the primary is the role that Pennsylvania Gov. Ed Rendell (D) will play in the race. Although Rendell, a former Philadelphia mayor, has endorsed Hoeffel and pledged to help him, many observers question how energetically the governor will work against Specter, his close friend of almost 40 years.

“That was one of our concerns, that if Specter had lost, Rendell would have become much more engaged on the Democratic side in terms of fundraising and organization,” said one Republican source.

Meanwhile, Tuesday’s primaries also set up several potentially competitive races across the state.

In the suburban/urban Northeast Philadelphia 13th district, state Sen. Allyson Schwartz (D) and wealthy ophthalmologist Melissa Brown (R) will square off in November in the race to succeed Hoeffel.

Schwartz edged out former National Constitution Center President Joe Torsella 52 percent to 48 percent in the Democratic primary.

Brown, who has run for the seat three times before, beat out two opponents for her party’s nomination, garnering 39 percent.

Both parties made their case Wednesday for how each candidate is better positioned than the other to win the swing seat in November, and Republicans argued that opposing Schwartz, rather than Torsella, improves their chances of winning.

Schwartz faced residency issues early in the primary, and eventually moved into the district. She and Brown share the same base in Montgomery County and Republicans note that Schwartz, unlike Torsella, has a voting record to be scrutinized.

A Democratic analysis of the primary results found that Schwartz won more votes in Philadelphia than Brown and party strategists are convinced she can win Northeast Philadelphia convincingly in the fall.

Regardless, the November contest is expected to be a costly and closely watched affair.

Schwartz, with the help of EMILY’s List, raised and spent about $1.4 million in the primary. Brown has contributed significant resources to her previous bids and is expected to do so again this time around.

In the 15th district race to replace Toomey, state Sen. Charlie Dent (R) and real estate developer Joe Driscoll (D) will square off in November. Driscoll, a late entrant in the race, has also been dogged by the residency issue, with Republicans criticizing him for accepting more donations from New York than from individuals within the district.

Dent, a Lehigh Valley native, has represented about two-thirds of the district in the state Legislature and Republicans argue that his moderate profile fits the Allentown-based swing district. Al Gore narrowly won the district in the 2000 presidential contest.

In the sprawling central Pennsylvania 17th district, attorney Scott Paterno emerged from the six candidate Republican field in the race to challenge Rep. Tim Holden (D).

Holden defeated Rep. George Gekas (R) in 2002, although the district favors Republicans. Bush would have won 55 percent in the redrawn district in 2000.

Paterno received nearly 27 percent of the vote in a crowded primary. The 31-year-old son of Penn State Football Coach Joe Paterno, traveled to Washington, D.C., on Wednesday to meet with party officials. The Democratic Congressional Campaign Committee, meanwhile, wasted little time in seizing on the controversial columns Paterno wrote in college, all of which were aired publicly during the primary.

“The Republicans woke up this morning with an unelectable candidate whose strange beliefs put him way out of the mainstream,” said DCCC Chairman Robert Matsui (Calif.). “Are Republicans in Pennsylvania and Washington really going to support a candidate who supports drug legalization and believes a U.S. president was a serial killer?”

Meanwhile, in Tuesday’s most surprising result, Rep. Bill Shuster (R) narrowly staved off a primary challenge from political neophyte Michael DelGrosso.

Shuster won 53 percent, a result some observers chalked up to lingering dissatisfaction in the district stemming from his path to Congress. Shuster was elected to replace his father, former Rep. Bud Shuster (R), in a contentious 2001 special election.