Rick: Miguel Estrada is a highly qualified attorney whose nomination is supported fully by the American Bar. It is true, as someone else said on this thread, that he has not been a judge, but if that was the qualification required there probably would not be very many judges today. Chief Justice Rhenquist, was never a judge before he was appointed to the Supreme Court by Richard Nixon. I'd venture a guest that many lawyers who never served as a judge have been appointed to the federal bench.
The Democrats object to Miguel Estrada because they suspect that he is too conservative. That's the sum of it.
When his nomination was presented to the U. S. Senate for confirmation, he received more than enough votes for confirmation from Republicans and some Democrats. However, since the Democrats have been involved in a filabuster attempting to block his appointment, it took more than a majority vote to break the filabuster and confirm him. Therefore he was not confirmed at that time. I'm sure the Republicans will not abandon him for the present anyway.
It's really rather simple. The Republicans want to see courts presided over by judges who share their political philosophy, and the Democrats want to see the courts presided over by judges that espouse theirs.
If Estrada is confirmed, he will be the first Hispanic appointed to the second highest court in the land. The Democrats fear that if he is confirmed the next move by some Republican president will be to appoint him to the Supreme Court. That to them would be disastrous.