Pennsylvania Senator Arlen Specter has now stretched his lead over Democratic Primary rival Joe Sestak to 21 points, his biggest lead yet. But, he’s still just barely over the 50% mark in terms of support.

A new Rasmussen Reports telephone survey of likely Pennsylvania Democratic Primary voters finds Specter ahead of Sestak 53% to 32%. Four percent (4%) prefer some other candidate, and 11% are undecided.

Last month, Specter led Sestak, a Philadelphia-area congressman, by 13 points, 48% to 35%.

Specter’s support has fallen between 46% and 51% in the earlier polls. So the new finding is his best showing yet, and gives the longtime incumbent senator, who switched parties last year because of a Republican Primary challenge, some breathing room. Incumbents who fall below 50% on a consistent basis are viewed as vulnerable.

In the four previous Rasmussen Reports polls on the race, Sestak’s support has ranged from 32% to 42%. He was most competitive in October when the numbers showed Specter with 46% of the vote and Sestak at 42%.

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Specter now carries male primary voters by 18 points and female voters by 23.

Sixty-eight percent (68%)of Pennsylvania Democrats favor the health care plan proposed by President Obama and leaders of their party, while 28% oppose it. Those numbers include 38% who Strongly Favor the plan and 19% who Strongly Oppose it. These findings are basically unchanged from December.

While Sestak opted to challenge Specter from the political left, arguing that he was the “real” Democrat in the race, 70% of those who Strongly Favor the health care plan support Specter, while 56% who Strongly Oppose it support his opponent.

Sixty-four percent (64%) of liberal Democratic voters and 50% of party moderates prefer Specter, while conservatives are almost evenly divided.

Specter was a Republican senator for nearly 30 years, but he switched parties last April just after a Rasmussen Reports poll showed him trailing conservative challenger Pat Toomey by 21 points in a state GOP Primary match-up. He acknowledged becoming a Democrat in part out of fear of losing the party primary.

Obama, Pennsylvania Governor Edward Rendell and other top Democrats welcomed Specter because it strengthened their party’s majority in the Senate, and they promised to campaign for his reelection. But Sestak, a retired Navy admiral, still opted for a primary challenge.

Rasmussen Reports will release new numbers tomorrow pitting Toomey, the likely GOP Senate candidate, against both Specter and Sestak.

Twenty-seven percent (27%) of Democratic Primary voters now have a very favorable opinion of Specter, while 13% view him very unfavorably. Only two percent (2%) have no opinion of Specter.

Sestak is seen very favorably by 16% and very unfavorably by six percent (6%). But 23% still don’t know enough about Sestak to venture even a soft favorable or unfavorable opinion of him.

At this point in a campaign, Rasmussen Reports considers the number of people with strong opinions more significant than the total favorable/unfavorable numbers.

Rasmussen Reports just released a New York Democratic Primary survey matching incumbent Senator Kirsten Gillibrand against potential challenger Harold Ford, Jr.

New data also has been released on Senate races in Arkansas, Colorado, Illinois, Connecticut, Missouri, Nevada, New Hampshire, North Dakota, Pennsylvania, Ohio, Texas, Florida. and California.

Election night polling also been released from yesterday's special Senate election in Massachusetts.

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