Republican Sen. Arlen Specter, a moderate who has often clashed with the Bush administration and his fellow GOP lawmakers, plans to seek a sixth term in 2010.

Specter, 77, will formally begin his re-election campaign April 4 at a Philadelphia fundraiser, Scott Hoeflich, a Specter spokesman, confirmed on Monday.

A former Philadelphia prosecutor dubbed "Snarlin' Arlen," Specter has never shied from controversial issues.

In 2004, Specter's sharp political analysis nearly cost him the chairmanship that he coveted after years on the Senate Judiciary Committee.

Shortly after his election to a fifth term, Specter angered conservatives long suspicious of him by warning a just re-elected President Bush that anti-abortion judges would have a difficult time winning Senate confirmation, given Democratic opposition.

Social conservatives demanded that Senate GOP leaders deny Specter the chairmanship. Only his extraordinary public pledge to give Bush's nominees quick hearings and early votes, regardless of their views on abortion, spared Specter the ignominy of a chairmanship denied.

From his seat on Judiciary, Specter has participated in the Senate confirmation hearing of every sitting Supreme Court Justice except Justice John Paul Stevens. He was chairman of the committee during the confirmation hearings for Chief Justice John Roberts and Supreme Court Justice Samuel Alito.

He is now the top Republican on the panel.

In 2005, while undergoing chemotherapy treatment for Hodgkin's disease, he challenged the administration over federal funding for embryonic stem-cell research. He has since said that he is fully recovered.

The scheduled fundraiser was first reported by The Patriot-News of Harrisburg.

Specter's last re-election campaign was the toughest of his career. He barely defeated former Rep. Pat Toomey in a primary race in which his age was contrasted with that of his youthful-looking opponent, but Specter went on to handily win the general election.