John Kemp Great Debate– John Kemp is a Reuters columnist. The views expressed are his own –

Senator Arlen Specter’s decision to cross the aisle and join the Democratic Party hastens the extinction of moderate Republicans in the north-east and symbolizes their deep problems.

But it does not change the legislative landscape.

On the most contentious parts of the president’s program — cap-and-trade emissions program, healthcare and Social Security — the key divisions are among Democrats rather than between the parties. Specter’s defection numerically swells the party’s ranks but in practice brings the administration no closer to the magic 60 votes it needs to push through ambitious reform proposals.

Senate tradition allows even a single senator to block passage of legislation by filibustering it. Since time is precious, even the threat of a filibuster is usually enough to stop legislation in its tracks.

Rule XXIII (cloture) curtails the right of unlimited debate and caps further debate at 30 hours. But it can only be invoked with agreement of 60 senators (three-fifths of the chamber’s total membership).