As of now, you don't have to declare your party affiliation before voting in a primary in Texas. We have an open primary. HB 1821 seeks to change that.
Vince Leibowitz at Capitol Annex thinks this would benefit the Texas Republican Party:
[the] bill is clearly aimed to cut Texas Democrats off at the knees after an election cycle in which primary participation exceeded two million and at a time when the party is on the rise statewide after 2008’s record turnout.
The bill, HB 1821, would require voters to declare a party affiliation at the time of registration, or be declared “independent.” Those who do not would be able to change their party affiliation. A 30 day waiting period would apply in odd-numbered years. In even numbered years (typically election years), a voter would have to change party affiliation–even if they are already registered–at least 31 days prior to the primary; the change would take effect after 30 days, meaning that the change would take effect the day before the primary and after early voting has closed, resulting in new voters being shut out of early voting.
It isn’t hard to see how the change would hurt Democrats in Texas. Given that more and more people are voting Democratic who were previously independent or Republican, it would mean that more people would have to cross a hurdle before being able to vote in the party’s primaries.
The change would also mean that Democratic campaigns in the primary would be spending a lot of money not just on voter registration but making sure that voters who did want to vote in those primaries changed their affiliation before the primary.
