See below: they're foucusing on GOP senators because they know they know that, unlike health care, they can't use reconciliation for amnesty:
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Roll Call
Groups Increase Pressure for Immigration Bill
March 22, 2010
By Anna Palmer and Jessica Brady
Roll Call Staff

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Immigration reform advocates are keeping the heat on key Senators to act quickly on a new legislative package, saying time is short to get anything done this year.

Despite scoring a couple of successes in recent days — the unveiling of a bipartisan proposal by Sens. Charles Schumer (D-N.Y.) and Lindsey Graham (R-S.C.) last week followed by a large rally Sunday on the National Mall — reform advocates are worried it will get increasingly difficult to move legislation with each day that passes before the midterm elections.

“The moment of truth will come soon,â€